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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 /ɪmˈpæsəbəl/

impassable

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

inconceivable

Meaning
Impossible to imagine or believe; beyond comprehension.
Example
It seemed inconceivable that she could finish the marathon after such little training.
C1 adjective /ˌɪnəˈtɛntɪv/

inattentive

Meaning
not paying attention; lacking focus or concentration
Example
The inattentive student missed several important instructions.
C1 noun ɪnˈdʌldʒəns

indulgence

Meaning
The practice of allowing oneself to enjoy the pleasure of something.
Example
His indulgence in junk food affected his health.
C1 adjective /ˌɪnstənˈteɪniəs/

instantaneous

Meaning
Happening immediately, without any delay.
Example
The change in temperature was almost instantaneous.
C1 adjective /ɪˈlɪsɪt/

illicit

Meaning
Forbidden by law, rules, or custom.
Example
The police shut down the illicit drug operation.
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 adjective /ˌɪnækˈsɛsəbl/

inaccessible

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

illegitimate

Meaning
Not authorized by law, rules, or standards; not recognized as valid.
Example
The court ruled the contract to be illegitimate.
C1 adjective /ˌɪnkɒmprɪˈhɛnsəbl/

incomprehensible

Meaning
Impossible or very difficult to understand.
Example
The professor’s lecture was so fast it became incomprehensible.
C1 adjective /ɪnˈfæləbl̩/

infallible

Meaning
incapable of making mistakes or being wrong
Example
Her judgment seemed infallible in matters of finance.
C1 verb /ɪnˈtruːd/

intrude

Meaning
To enter a place or situation where one is not wanted or not invited.
Example
He felt embarrassed to intrude on their private conversation.
C1 adjective /ˌɪnkəmˈpætəbl/

incompatible

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

instigator

Meaning
A person who brings about or initiates something, usually something bad.
Example
The police identified him as the instigator of the violence.
C1 noun /ˌɪnkəmˌpætəˈbɪlɪti/

incompatibility

Meaning
the state of being unable to exist or work together harmoniously; lack of compatibility
Example
The project failed because of incompatibility between the two software systems.
C1 adjective /ˌɪn.səˈbɔːr.dɪ.nət/

insubordinate

Meaning
Defiant of authority; disobedient to orders.
Example
The insubordinate employee refused to follow the manager's instructions.
C1 adjective /ɪnˈveɪsɪv/

invasive

Meaning
Tending to spread harmfully, intruding aggressively, or encroaching beyond proper boundaries.
Example
The doctor explained that the procedure was minimally invasive.
C1 adjective /ˌɪn.dɪˈspen.sə.bəl/

indispensable

Meaning
Absolutely necessary, essential
Example
Water is indispensable for human survival.
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 verb ɪˈmɜːrs

immerse

Meaning
To involve oneself deeply in a particular activity or interest; to surround completely.
Example
Immerse yourself in the new language.
C1 adjective /ɪnˈfriːkwənt/

infrequent

Meaning
Happening rarely or not often.
Example
He made infrequent visits to his hometown after moving abroad.
C1 adjective /ɪnˈkɒmp(ə)rəbl/

incomparable

Meaning
Without an equal in quality or excellence; matchless.
Example
The view from the mountain was incomparable.
C1 verb /ɪmˈploʊd/

implode

Meaning
To collapse or burst inward violently.
Example
The old building imploded during the demolition.
C1 noun /ˌɪrɪˌspɒnsəˈbɪləti/

irresponsibility

Meaning
lack of responsibility or accountability; failure to act in a reliable or sensible way
Example
The accident was caused by his irresponsibility at work.
C1 adjective /ˈaɪərnˌklæd/

ironclad

Meaning
impossible to change, break, or dispute; very strong and firm
Example
The company had an ironclad contract with its suppliers.
C1 noun /ˈɪrɪtənt/

irritant

Meaning
something that causes irritation or discomfort
Example
Dust is a common irritant for people with allergies.
C1 noun /ˌɪntɪˈmeɪʃən/

intimation

Meaning
A subtle or indirect hint or suggestion.
Example
She gave no intimation of her plans.
C1 verb /ˌɪntəˈsɛkt/

intersect

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

inexplicable

Meaning
Impossible to explain or understand.
Example
The sudden disappearance of the file was inexplicable.
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 /ɪnˈfleɪtɪd/

inflated

Meaning
excessively increased in size, value, or importance; filled with air or gas
Example
The company faced criticism for its inflated prices.
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 /ˌɪnədˈmɪsəbl̩/

inadmissible

Meaning
Not allowed, especially as evidence in a court of law.
Example
The judge ruled the evidence inadmissible.
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 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 /ˌɪnsərˈmaʊntəbl/

insurmountable

Meaning
Too great to be overcome.
Example
The explorers faced insurmountable odds in the desert.
C1 adjective /ɪnˈtɜːrmənəbl/

interminable

Meaning
Seemingly endless; having no apparent end.
Example
The meeting felt interminable and exhausting.
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 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 noun /ˈɪntrɪkəsi/

intricacy

Meaning
the quality of being very detailed or complicated
Example
The intricacy of the design impressed everyone.
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 adjective /ˌɪməˈtɪəriəl/

immaterial

Meaning
Not important or relevant; lacking physical substance.
Example
The judge ruled that the argument was immaterial to the case.
C1 verb /ɪmˈpɑːrt/

impart

Meaning
to make information known; to bestow or communicate
Example
The teacher imparted valuable knowledge to her students.
C1 noun ˌɪnˈsaɪtfʊlnɪs

insightfulness

Meaning
The capacity to gain an accurate and deep intuitive understanding of a person or thing.
Example
His insightfulness helped solve the complex problem.
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 verb /ˈɪnkjʊˌbeɪt/

incubate

Meaning
to keep eggs, cells, or organisms at the right conditions for development
Example
The hen will incubate the eggs until they hatch.
C1 adverb /ɪnˈvɜːrs.li/

Inversely

Meaning
in a way that is opposite or contrary; in reverse order or manner
Example
The price of goods is inversely related to their availability in the market.
C1 adjective /ɪnˈtrɛpɪd/

intrepid

Meaning
fearless and adventurous, often used humorously
Example
The intrepid traveler explored the jungle alone.
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 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 verb /ɪnˈkrɪmɪˌneɪt/

incriminate

Meaning
to make someone appear guilty of a crime or wrongdoing
Example
The new evidence could incriminate the suspect in the robbery case.
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 noun /ˌɪnˈdɪsəplɪn/

indiscipline

Meaning
lack of discipline or self-control; failure to obey rules or maintain order
Example
Indiscipline among the students disrupted the learning environment.
C1 adjective /ˌɪrɪˈspɛktɪv/

Irrespective

Meaning
without taking something into account; regardless of
Example
All students will be treated equally, irrespective of their background.
C1 noun /ˈɪŋ.krə.mənt/

Increment

Meaning
an increase or addition, especially one of a series on a fixed scale; a regular increase in salary
Example
He received an annual increment of five percent in his salary this year.
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 /ɪmˈpɛn.ɪ.trə.bəl/

impenetrable

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

irreversible

Meaning
Not able to be undone, changed, or reversed.
Example
The damage caused by the fire was irreversible.
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 /ˈɪntrɪkət/

intricate

Meaning
very detailed, complex, and difficult to understand
Example
The artist created an intricate pattern on the wall.
C1 adjective /ɪnˈdɪɡnənt/

indignant

Meaning
Feeling or showing anger at something considered unfair or unjust.
Example
She was indignant when accused of cheating.
C1 adjective /ɪnˈveə.ri.ə.bəl/

Invariable

Meaning
never changing; constant; always the same
Example
His invariable routine includes morning exercise and coffee.
C1 adjective /ɪnˈkɒmpɪtənt/

incompetent

Meaning
Lacking the necessary ability or skills to do something successfully.
Example
He was fired because he was incompetent at his job.
C1 verb /ɪnˈfɛst/

infest

Meaning
to overrun in large numbers, especially so as to cause damage or discomfort
Example
The old house was infested with rats.
C1 adjective ɪnˈsaɪ.sɪv

incisive

Meaning
Intelligently analytical and clear-thinking; showing clear thought and good understanding of what is important.
Example
His incisive comments revealed the core issues.
C1 adjective /aɪˈreɪt/

irate

Meaning
Extremely angry.
Example
The customer became irate after waiting for hours.
C1 adjective /ɪˈnɔːɡjərəl/

Inaugural

Meaning
marking the beginning of something; first; opening
Example
The president delivered his inaugural speech.
C1 noun /ˈɪd.i.əm/

idiom

Meaning
A group of words whose meaning is different from the meanings of the individual words.
Example
The idiom 'break the ice' means to start a conversation in a friendly way.
C1 noun /ˈɪm.pæs/

impasse

Meaning
A situation where no progress is possible.
Example
The negotiations reached an impasse, delaying the agreement.
C1 adjective ɪnɪɡˈzɔːstɪbəl

inexhaustible

Meaning
Unable to be used up or worn out; limitless.
Example
His inexhaustible energy allowed him to work tirelessly on his projects.
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 /ɪnˈsteɪbəl/

instable

Meaning
not stable; liable to change, collapse, or fail; lacking firmness or reliability
Example
The instable political situation worried foreign investors.
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 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 adjective ɪn ˈfleɪ.ʃə.ner.i

inflationary

Meaning
Characterized by or tending to cause monetary inflation; relating to rising prices.
Example
The government took measures to control inflationary pressures.
C1 noun /ˌɪnkjʊˈbeɪʃən/

incubation

Meaning
the process of keeping eggs, cells, or organisms in the right conditions for development
Example
The incubation of the eggs lasted for three weeks.
C1 noun ˌɪrɪˈvɜːrsəbl ˈdæmɪdʒ

irreversible damage

Meaning
Harm or injury that cannot be undone or repaired.
Example
Climate change causes irreversible damage to ecosystems.
C1 adjective /ɪˈmuː.və.bəl/

immovable

Meaning
Not able to be moved; fixed in position.
Example
Immovable barriers prevent change.
C1 noun /ɪnˈdem.nə.ti/

Indemnity

Meaning
security or protection against a loss or other financial burden; compensation for damage or loss
Example
The insurance company provided full indemnity for the damages caused by the flood.
C1 noun /ˌɪm.pəˈzɪʃ.ən/

Imposition

Meaning
the action or process of imposing something or of being imposed; an unfair burden or demand
Example
The imposition of new taxes without consultation angered the citizens greatly.
C1 noun /ˌɪnsɪɡˈnɪfɪkəns/

insignificance

Meaning
The quality of being unimportant or trivial.
Example
He felt a sense of insignificance in the vast universe.
C1 adjective /ˈɪmpətənt/

impotent

Meaning
Lacking power, ability, or strength to take effective action.
Example
The citizens felt impotent against the corrupt system.
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 /ˌaɪdiəˈlɪstɪk/

Idealistic

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

incidence

Meaning
the occurrence, rate, or frequency of something
Example
The incidence of road accidents has decreased this year.
C1 adjective /ɪnˈhjuːmən/

Inhuman

Meaning
lacking human qualities; cruel; brutal; savage
Example
The dictator's inhuman treatment of prisoners shocked the international community.
C1 verb ɪˈnɪʃɪeɪtɪd

initiated

Meaning
Caused a process or action to begin; started something.
Example
The project was initiated to enhance marine biodiversity.
C1 adjective /ɪmˈpɛtʃ.u.əs/

impetuous

Meaning
Acting or done quickly and without thought or care; impulsive.
Example
His impetuous decision led to unexpected consequences.
C1 noun /ɪnˈfɜːməri/

infirmary

Meaning
A hospital or place where the sick or injured are cared for.
Example
The school had an infirmary for minor injuries.
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 noun ˌɪn.fɪlˈtreɪ.ʃən

infiltration

Meaning
The process of entering or gaining access to an organization or place surreptitiously.
Example
Cybersecurity prevents infiltration into sensitive data.
C1 noun /ɪnˈkɜːrʒən/

incursion

Meaning
A sudden attack or invasion into a place or territory.
Example
The army repelled the enemy incursion at the border.
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 /ɪnˈvɜːrʒən/

inversion

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

inquest

Meaning
A legal or official inquiry, especially into the cause of a death.
Example
The inquest revealed the cause of the accident.
C1 adjective /ɪmˈpɑːrʃəl/

Impartial

Meaning
treating all sides fairly; unbiased; neutral
Example
A good judge must remain impartial throughout the trial.
C1 adjective /ˈɪrɪtəbl/

irritable

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

industriousness

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

insertion

Meaning
the action of inserting something into something else
Example
The insertion of the new data was completed successfully.
C1 verb ɪmˈbjuː

Imbue

Meaning
Mixing some idea or feeling deeply into someone's mind
Example
The teacher tried to imbue students with a love for learning.
C1 adjective /ɪmˈpæsɪv/

impassive

Meaning
Not showing or feeling emotion; expressionless.
Example
Despite the shocking news, she remained impassive.
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 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 adjective ˌɪdiəsɪnˈkrætɪk

idiosyncratic

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

interoperability

Meaning
The ability of computer systems or software to exchange and make use of information.
Example
The interoperability of different software systems ensures seamless workflow.
C1 noun ɪnˌɛv.ɪ.təˈbɪl.ɪ.ti

inevitability

Meaning
The quality of being certain to happen and impossible to avoid or prevent.
Example
The inevitability of change is a part of life.
C1 noun /ɪmˈpɜː.mə.nəns/

impermanence

Meaning
The state or fact of lasting for only a limited period of time.
Example
The philosophy of Buddhism emphasizes the impermanence of life.
C1 verb /ɪnˈsaɪt/

incite

Meaning
to provoke; to stir up
Example
His speech incited the crowd to violence.
C1 verb /ɪnˈterəˌɡeɪt/

interrogate

Meaning
to ask someone questions in a thorough and formal way; to question intensively
Example
The police interrogated the suspect for hours.
C1 adjective /ɪmˈpjʊər/

Impure

Meaning
contaminated; not clean; mixed with foreign substances
Example
The impure water caused health problems in the village.
C1 noun /ˌɪntərˈdʒɛkʃən/

interjection

Meaning
a word or phrase used to express sudden emotion or reaction, often standing alone
Example
Words like 'wow' and 'ouch' are common interjections in English.
C1 noun /ˈɪn.floʊ/

inflow

Meaning
The movement of things such as money, people, or water into a place.
Example
The inflow of tourists boosted the local economy.
C1 adjective /ɪnˈflæməˌtɔːri/

inflammatory

Meaning
causing inflammation in the body; arousing anger, hostility, or strong emotion
Example
His inflammatory speech triggered protests across the city.
C1 adjective /ɪˈnænɪmət/

inanimate

Meaning
not alive, especially not in the manner of animals and humans; lifeless
Example
The museum was full of inanimate objects from ancient times.
C1 noun /ˌɪntəkəˈnɛktɪvɪti/

interconnectivity

Meaning
The state of being interconnected or the degree of connectivity between components
Example
The rise of interconnectivity has transformed global communication.
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 adjective /ˌɪnədˈvɜːrtənt/

inadvertent

Meaning
Not resulting from or achieved through deliberate planning; unintentional.
Example
Her inadvertent mistake caused a delay in the project.
C1 noun /ɪnˈsɒm.ni.ə/

Insomnia

Meaning
habitual sleeplessness; inability to sleep; the condition of being unable to sleep
Example
Her chronic insomnia made it difficult for her to concentrate during work hours.
C1 adjective /ˌɪndɪˈvɪzəbl/

indivisible

Meaning
Not able to be divided or separated.
Example
The country's unity was indivisible despite differences.