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C1 noun /ˈaɪsətəʊp/

isotope

Meaning
Atoms of the same element with equal number of protons but different numbers of neutrons.
Example
Carbon-14 is a radioactive isotope used in dating fossils.
C1 adjective ɪnˈdʒiː.ni.əs

ingenious

Meaning
Showing inventiveness and skill; clever, original, and inventive.
Example
His ingenious ideas transformed modern physics.
C1 noun ˌɪn.toʊˈneɪ.ʃən

intonation

Meaning
The rise and fall of tone in speech and pronunciation.
Example
Proper intonation enhances the clarity of a speech.
C1 adjective /ɪnˈkrɛdjələs/

incredulous

Meaning
unwilling or unable to believe something
Example
He gave her an incredulous look when she claimed she could fly.
C1 noun /ɪnˈkɒmpɪtəns/

incompetence

Meaning
Lack of ability, skill, or effectiveness in doing something.
Example
The project failed due to the manager’s incompetence.
C1 noun ˈɪn.fər.əns

inference

Meaning
A conclusion reached on the basis of evidence and reasoning; logical deduction.
Example
Scientists make inferences based on experimental data.
C1 noun /ɪˌluːmɪˈneɪʃən/

illumination

Meaning
the act of lighting something or making something clear
Example
The street was filled with bright illumination from the lamps.
C1 noun /ˈɪntərˌluːd/

interlude

Meaning
A short period of time that comes between two events; an interval.
Example
There was a brief musical interlude between the two acts of the play.
C1 adjective /ˌɪn.dɪˈspen.sə.bəl/

indispensable

Meaning
Absolutely necessary, essential
Example
Water is indispensable for human survival.
C1 adverb ɪnˈtrɪn.zɪ.kli

intrinsically

Meaning
In a way that is naturally or inherently part of something's essential nature.
Example
Some people are intrinsically motivated to learn.
C1 noun /ɪnˈvɜːrʒən/

inversion

Meaning
A reversal of the normal order, position, or relationship.
Example
In poetry, inversion is often used to create emphasis.
C1 adjective /ɪnˈtɜːrmənəbl/

interminable

Meaning
Seemingly endless; having no apparent end.
Example
The meeting felt interminable and exhausting.
C1 verb /ˈɪmprəvaɪz/

improvise

Meaning
To create or perform something spontaneously without preparation.
Example
The actor had to improvise his lines when he forgot the script.
C1 adjective /ɪnˈkɒŋɡruəs/

incongruous

Meaning
Not fitting in with or suitable for the surroundings.
Example
His jokes were incongruous during the serious meeting.
C1 noun /ɪmˈpɛd.ɪ.mənt/

impediment

Meaning
A hindrance or obstruction in doing something; something that prevents progress.
Example
Lack of funds is a major impediment to the project.
C1 noun ˌɪndɪˈspensəbl̩ ˈkɒmpɪtənsi

indispensable competency

Meaning
Essential skill or ability that cannot be done without; a crucial capability that is absolutely necessary.
Example
Digital literacy is an indispensable competency in the 21st century.
C1 noun /ɪnˈdʌstriəsnəs/

industriousness

Meaning
The quality of being hardworking and diligent.
Example
His industriousness earned him a promotion at work.
C1 noun /ˌɪnklɪˈneɪʃən/

inclination

Meaning
A natural tendency or urge to act or feel in a particular way.
Example
She has an inclination towards helping others.
C1 adjective /ˌɪnækˈsɛsəbl/

inaccessible

Meaning
unable to be reached, entered, or understood
Example
The remote village is almost inaccessible during the rainy season.
C1 adjective /ɪmˈpæsəbəl/

impassable

Meaning
Impossible to travel along or over.
Example
The road became impassable after the heavy snowfall.
C1 noun /ˌɪnkənˈsɪstənsi/

inconsistency

Meaning
The quality of being contradictory, irregular, or not staying the same.
Example
There is an inconsistency between his words and actions.
C1 noun /ˌɪnkwɪˈzɪʃən/

inquisition

Meaning
A prolonged and intense questioning or investigation, often harsh in nature.
Example
The journalist faced an inquisition from the panel about his sources.
C1 adjective /ɪnˈkwɪzɪtɪv/

inquisitive

Meaning
Curious or eager to learn and ask questions.
Example
The inquisitive child asked endless questions about the stars.
C1 adjective /ˈɪndɪɡoʊ bluː/

Indigo-blue

Meaning
a deep blue color with a purplish tinge
Example
The indigo-blue sky looked magnificent at twilight.
C1 adjective; noun /ɪˈmɔːrtl/

immortal

Meaning
living forever; never dying; lasting forever in fame or memory
Example
Legends often describe heroes as immortal beings who never die.
C1 adjective /ɪnˈɔːdəbl/

inaudible

Meaning
Unable to be heard.
Example
Her voice was almost inaudible over the noise.
C1 noun /ˈɪn.tə.lekt/

intellect

Meaning
The ability to think, reason, and understand objectively.
Example
Her intellect is admired by all her colleagues.
C1 noun ˌɪnsjʊˈleɪʃən

insulation

Meaning
Material used to prevent heat, sound, or electricity from being transmitted from one area to another.
Example
Insulation of food storage areas can prevent pests from entering.
C1 noun (plural) /ˈɪndɪˌsiːz/

indices

Meaning
plural form of index; statistical measures or reference points used for comparison
Example
Stock market indices fell sharply after the announcement.
C1 adjective /ɪnˈdefɪnət/

Indefinite

Meaning
not clearly defined; uncertain; vague; unlimited in time
Example
The meeting has been postponed for an indefinite period.
C1 adjective /ɪnˈvɑːlənˌtɛri/

involuntary

Meaning
done without will or conscious control
Example
He gave an involuntary shiver when he heard the loud noise.
C1 noun /ˈɪmɪdʒəri/

imagery

Meaning
visually descriptive or figurative language, especially in a literary work
Example
The poet's use of imagery brought the scene to life.
C1 verb /ɪnˈvoʊk/

invoke

Meaning
to call upon a higher power, authority, or principle for help, support, or justification
Example
The lawyer invoked the constitution to defend his client.
C1 adjective /ɪmˈprɒmp.tjuː/

impromptu

Meaning
Done without being planned, organized, or rehearsed; spontaneous.
Example
He gave an impromptu speech at the event.
C1 adjective ˌɪdiəsɪnˈkrætɪk

idiosyncratic

Meaning
Relating to idiosyncrasy; peculiar or individual.
Example
His idiosyncratic behavior set him apart from his colleagues.
C1 noun /ˌɪnflɛksɪˈbɪlɪti/

inflexibility

Meaning
the quality of being rigid or unwilling to change; lack of adaptability
Example
The inflexibility of the rules caused unnecessary delays.
C1 adjective /ɪˈnɛpt/

inept

Meaning
lacking skill or ability; clumsy or incompetent
Example
His inept handling of the situation made it worse.
C1 noun ˌɪnfɛsˈteɪʃən

infestation

Meaning
The presence of an unusually large number of insects or animals in a place, causing damage or disease.
Example
The infestation of ants in the kitchen led to immediate pest control measures.
C1 noun /ˌɪn.trəˈvɜːr.ʒən/

introversion

Meaning
The tendency to be inward-looking and shy, preferring solitary activities.
Example
Her introversion made it difficult for her to enjoy large parties.
C1 adjective /ˌɪnkəmˈpætəbl/

incompatible

Meaning
Not able to exist or work together without conflict.
Example
Their personalities were completely incompatible.
C1 adjective /ˌɪnkənˈsɪdərət/

inconsiderate

Meaning
Thoughtless; showing a lack of concern for others.
Example
It was inconsiderate of him to play loud music late at night.
C1 noun /ɪnˈtɛstɪn/

intestine

Meaning
The part of the digestive system where food is digested and absorbed, extending from the stomach to the anus.
Example
The doctor explained how nutrients are absorbed in the small intestine.
C1 noun /ɪnˈtruː.ʒən/

intrusion

Meaning
Unauthorized entry into a system or place without permission
Example
The company detected an intrusion in its server.
C1 adjective /ˌaɪdiəˈlɪstɪk/

Idealistic

Meaning
pursuing high principles or ideals; unrealistically optimistic
Example
She has an idealistic view of changing the world.
C1 adjective /ɪnˈvɪnsəbl/

invincible

Meaning
too powerful to be defeated or overcome
Example
The team seemed invincible after winning ten matches in a row.
C1 adjective /ˌɪndɪˈskrɪmɪnət/

indiscriminate

Meaning
Done without careful judgment; random or haphazard.
Example
The indiscriminate use of pesticides harms the environment.
C1 adjective ɪnˈkʌm.bənt

incumbent

Meaning
necessary for (someone) as a duty or responsibility.
Example
It is incumbent upon all citizens to obey the law.
C1 adjective /ˌɪnhɒˈspɪtəbl/

inhospitable

Meaning
Harsh and difficult to live in; unfriendly or unwelcoming to guests.
Example
The desert is an inhospitable environment for most plants.
C1 noun /ɪmˈprɪzənmənt/

imprisonment

Meaning
the act of putting someone in prison; the state of being confined as punishment
Example
He faced imprisonment for violating the court order.
C1 noun /ɪmˌpɒsəˈbɪlɪti/

impossibility

Meaning
the state or fact of being impossible; something that cannot be done or achieved
Example
Finishing the project in one day was an impossibility.
C1 verb /ɪmˈbɛd/

imbed

Meaning
To fix firmly or deeply in a surrounding mass.
Example
The journalist imbedded himself with the troops to report firsthand.
C1 adjective /ɪmˈplɔːzəbl/

implausible

Meaning
Not seeming reasonable or probable; unlikely to be true.
Example
The story she told was so implausible that no one believed her.
C1 noun /ɪnˈfjuːʒən/

infusion

Meaning
the act of introducing something into another; a drink made by steeping leaves or herbs in liquid
Example
She prepared a calming infusion of chamomile tea.
C1 adjective /ɪˈmoʊbəl/

immobile

Meaning
unable to move or be moved
Example
After the accident, he was immobile for weeks.
C1 verb /ˌɪntərˈsɛpt/

intercept

Meaning
to stop, catch, or seize something on its way from one place to another
Example
The police intercepted the stolen car before it crossed the border.
C1 noun /ˌɪn.səˌbɔːr.dɪˈneɪ.ʃən/

insubordination

Meaning
Defiance of authority; refusal to obey orders.
Example
The soldier was punished for insubordination during the mission.
C1 verb /ɪmˈpiːtʃ/

impeach

Meaning
To charge a public official with misconduct while in office.
Example
The parliament voted to impeach the president.
C1 verb /ɪmˈpɜːrsəneɪt/

impersonate

Meaning
to pretend to be another person in order to entertain or deceive
Example
He was arrested for trying to impersonate a police officer.
C1 noun /aɪˈdiː.ə.lɪ.zəm/

Idealism

Meaning
the pursuit of high principles and noble goals; unrealistic pursuit of perfection
Example
His youthful idealism motivated him to join various social justice movements.
C1 noun /ɪnˈsæn.ə.ti/

Insanity

Meaning
the state of being seriously mentally ill; extreme foolishness or irrationality
Example
The defense lawyer argued that his client was not guilty by reason of insanity.
C1 adjective ˌɪn.təˈdɪs.ɪ.plɪ.nər.i

interdisciplinary

Meaning
Relating to more than one branch of knowledge.
Example
The interdisciplinary approach in education enhances problem-solving skills.
C1 adjective /ˌɪnəˈfɛnsɪv/

inoffensive

Meaning
Not likely to offend or provoke; harmless.
Example
His inoffensive remarks kept the conversation friendly.
C1 adjective /ɪmˈpæsɪv/

impassive

Meaning
Not showing or feeling emotion; expressionless.
Example
Despite the shocking news, she remained impassive.
C1 adverb /ˈɪnwərdli/

inwardly

Meaning
In the mind or spirit; not outwardly visible.
Example
She smiled outwardly but inwardly she was worried.
C1 noun /ˈɪŋklɪŋ/

inkling

Meaning
A slight idea, hint, or suspicion about something.
Example
She had an inkling that he was planning a surprise.
C1 noun ˌɪntədɪˈpɛndənsi

interdependency

Meaning
The dependence of two or more people or things on each other.
Example
The interdependency of the countries led to greater cooperation.
C1 adverb /ˌɪnsɪˈdɛntəli/

incidentally

Meaning
by the way; used to introduce a remark not directly related to the main subject
Example
Incidentally, did you know that she studied in Paris?
C1 adjective /ɪˈnɔːɡjərəl/

Inaugural

Meaning
marking the beginning of something; first; opening
Example
The president delivered his inaugural speech.
C1 adjective /ɪnˈdʌstriəs/

industrious

Meaning
Hardworking and diligent in one's duties.
Example
She is an industrious student who always completes her assignments on time.
C1 noun ˌɪnfrəˈstrʌktʃərəl dɪˈveləpmənt

infrastructural development

Meaning
The process of building and improving the basic physical and organizational structures and facilities needed for the operation of a society or enterprise.
Example
Infrastructural development is necessary for efficient recycling.
C1 adjective /ɪnˈtjuː.ɪ.tɪv/

intuitive

Meaning
Based on instinctive understanding rather than reasoning.
Example
The software has an intuitive design that makes it easy to use.
C1 adjective /ˌɪndɪˈvɪzəbl/

indivisible

Meaning
Not able to be divided or separated.
Example
The country's unity was indivisible despite differences.
C1 verb /aɪˈdiː.ə.laɪz/

idealize

Meaning
To regard or represent something as perfect or better than it really is.
Example
Children often idealize their parents.
C1 adjective /ˌɪrɪˈprɛsəbl̩/

irrepressible

Meaning
Impossible to control or restrain.
Example
She had an irrepressible urge to laugh during the speech.
C1 noun /ˌaɪ.diˈɒl.ə.dʒi/

ideology

Meaning
A system of ideas and ideals, especially one that forms the basis of economic or political theory and policy.
Example
Democracy is based on the ideology of freedom and equality.
C1 noun /ɪnˈdʒʌŋkʃən/

injunction

Meaning
A judicial order restraining a person from an action or compelling a certain act.
Example
The court issued an injunction to stop the company from polluting the river.
C1 adjective /ɪnˈvɜːs/

Inverse

Meaning
opposite in order, nature, or effect; reversed
Example
There is an inverse relationship between price and demand.
C1 adjective, noun /ˈɪnvəlɪd/ (adjective), /ɪnˈvælɪd/ (noun)

invalid

Meaning
Not valid; having no force or effect. Also refers to a person too weak or ill to care for themselves.
Example
The court declared the contract invalid.
C1 verb /ɪɡˈnaɪt/

ignite

Meaning
To catch fire or cause to catch fire; to start or arouse.
Example
The sparks ignited the dry leaves in seconds.
C1 adjective /ɪnˈdʌktɪv/

inductive

Meaning
characterized by the inference of general laws from particular instances; reasoning from specific cases to general principles
Example
The scientist used inductive reasoning to form a hypothesis.
C1 adjective /ɪnˈɛlɪdʒəbəl/

Ineligible

Meaning
not qualified or entitled to participate or be chosen; unsuitable
Example
Students with poor grades are ineligible for the scholarship program.
C1 adjective /ˌɪnɑːrˈtɪkjʊlət/

inarticulate

Meaning
Unable to express oneself clearly or effectively in speech or writing.
Example
She became inarticulate with emotion during the speech.
C1 noun /ɪnˈsɜːrdʒəns/

insurgence

Meaning
An act of rebellion or uprising against authority.
Example
The government struggled to contain the growing insurgence.
C1 adjective /ɪmˈpʌlsɪv/

impulsive

Meaning
acting suddenly without careful thought
Example
She made an impulsive decision to buy the car.
C1 verb /ɪnˈkɜːr/

incur

Meaning
To bring upon oneself something undesirable, usually as a result of actions.
Example
He incurred a fine for parking in the wrong place.
C1 noun /ˌɪnɪkˈspɪəriəns/

inexperience

Meaning
Lack of knowledge or skill in a particular activity or field.
Example
His inexperience in handling negotiations was evident.
C1 adjective /ˌɪndɪˈstɪŋkt/

indistinct

Meaning
Not clear or sharply defined; vague or blurred.
Example
The sound of voices was indistinct in the distance.
C1 adjective ɪnˈdel.ə.bəl

indelible

Meaning
Making marks that cannot be removed; impossible to forget or remove.
Example
Her speech left an indelible mark on the audience.
C1 noun /ˌɪnkɑːrˈneɪʃən/

incarnation

Meaning
the embodiment of a deity, spirit, or quality in a physical form
Example
The monk was considered the incarnation of compassion.
C1 noun ˌɪndɪˈspensəbl səˈpɔːrt

indispensable support

Meaning
Essential assistance that cannot be done without; absolutely necessary help.
Example
Volunteers provide indispensable support to communities.
C1 adjective /ɪmˈpɛn.ɪ.trə.bəl/

impenetrable

Meaning
Impossible to pass through or enter; impossible to understand.
Example
The dense forest was nearly impenetrable.
C1 verb /ɪmˈpɒvərɪʃ/

impoverish

Meaning
To make someone poor or reduce the quality or richness of something.
Example
The long war impoverished the entire nation.
C1 adjective /ˈɪnbɔːrn/

Inborn

Meaning
existing from birth; natural; inherent
Example
She has an inborn talent for music.
C1 verb /ɪˈmɔːrtəlaɪz/

immortalize

Meaning
to preserve the memory of someone or something forever
Example
The statue was built to immortalize the hero’s sacrifice.
C1 adjective /ɪnˈses.ənt/

incessant

Meaning
Continuing without pause or interruption; constant.
Example
The incessant noise from the construction site made it hard to concentrate.
C1 noun /ɪnˈsaɪtmənt/

incitement

Meaning
the action of provoking unlawful or violent behavior
Example
He was arrested for incitement to riot.
C1 adjective ɪnˈtrɪn.zɪk

intrinsic

Meaning
Belonging naturally; essential. Existing as a natural or basic part of something.
Example
Creativity is an intrinsic quality of a great artist.
C1 adjective /ɪˈledʒ.ə.bəl/

illegible

Meaning
Not clear enough to be read; unreadable.
Example
The doctor’s handwriting was almost illegible.
C1 noun /ˌɪnkəmˈpliːtnəs/

incompleteness

Meaning
the state of not being complete or whole; lack of necessary parts or fullness
Example
The incompleteness of the report caused delays in the project.
C1 adjective /ˈɪɡ.ni.əs/

igneous

Meaning
Formed through the cooling and solidification of magma or lava.
Example
Granite is an igneous rock commonly used in construction.
C1 noun /ˌɪmpɔːrˈteɪʃən/

importation

Meaning
The act of bringing goods or services into a country from abroad
Example
The importation of luxury goods is strictly regulated.
C1 adjective /ɪnˈsʌf.ər.ə.bəl/

insufferable

Meaning
Too extreme to bear; intolerable.
Example
The heat during the summer was insufferable.
C1 adjective /ˈɪrɪtəbl/

irritable

Meaning
easily annoyed or made angry
Example
She became irritable after working long hours without rest.
C1 noun /ɪnˈdjuːsmənt/

inducement

Meaning
something that persuades or encourages someone to do something
Example
The company offered a bonus as an inducement to employees.
C1 adjective /ˌɪməˈtɪəriəl/

immaterial

Meaning
Not important or relevant; lacking physical substance.
Example
The judge ruled that the argument was immaterial to the case.
C1 adjective ˌɪn.tə.dʒen.əˈreɪ.ʃən.əl

intergenerational

Meaning
Existing or occurring between different generations of a family.
Example
Intergenerational wisdom ensures cultural continuity.
C1 verb /ɪmˈpɛər/

impair

Meaning
to weaken or damage something, especially a function or ability
Example
Smoking can impair your lung capacity.
C1 adjective /ɪˈnɔːrdənət/

inordinate

Meaning
Excessive or unusually large in degree.
Example
She spent an inordinate amount of time on her phone.
C1 noun /ɪnˈdaɪtmənt/

indictment

Meaning
a formal charge or accusation of a serious crime
Example
The indictment accused the company of multiple violations.
C1 noun /ɪˌreɡjəˈlærɪti/

irregularity

Meaning
lack of regularity or consistency; a deviation from what is normal, expected, or orderly
Example
The audit revealed several financial irregularities in the company’s records.
C1 adjective /ɪnˈæk.tɪv/

Inactive

Meaning
not active; not working; dormant; idle
Example
The volcano has been inactive for over a century.
C1 adjective ɪmˈpɛkəbl

impeccable

Meaning
In accordance with the highest standards; faultless.
Example
His impeccable taste in fashion was admired by all his peers.
C1 noun /ɪnˈdɪɡ.nə.ti/

Indignity

Meaning
treatment or circumstances that cause one to feel shame or loss of dignity; humiliation
Example
She refused to suffer the indignity of being searched without proper cause.
C1 noun /ˌɪməˈrælɪti/

immorality

Meaning
the quality of being immoral; behavior that violates accepted moral standards
Example
The community condemned the immorality of the corrupt officials.
C1 noun /ɪˈmɜː.ʃən/

immersion

Meaning
Complete engagement in an environment or experience.
Example
Immersion in VR games makes users feel like they are inside the virtual world.
C1 adjective /ɪnˈsɒlvənt/

insolvent

Meaning
unable to pay debts owed
Example
The insolvent firm could not meet its financial obligations.
C1 verb /ɪnˈfleɪm/

inflame

Meaning
to arouse strong feelings or provoke anger; to cause inflammation in the body
Example
The politician's remarks served to inflame public opinion.
C1 verb /ˌɪntəˈsɛkt/

intersect

Meaning
to cross or pass through each other
Example
The two roads intersect at the city center.
C1 adjective /ˌɪndɪˈfɛnsəbl̩/

indefensible

Meaning
not able to be protected or justified
Example
The minister’s remarks were considered indefensible by the public.
C1 adjective ˌɪn.təˈstel.ər

interstellar

Meaning
Occurring or situated between stars.
Example
Scientists study interstellar travel possibilities.
C1 adjective /ɪnˈdiːsənt/

indecent

Meaning
not conforming to accepted standards of behavior or morality; improper or offensive
Example
He was warned for making an indecent remark during the meeting.