inefficacious
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C2 adjective ˌɪn.ɪ.fɪˈkeɪ.ʃəs

inefficacious

Meaning
Not producing any or the desired effect; ineffective.
Example
The treatment was inefficacious in curing the disease.
C2 adjective /ˌɪrɪˈkɒnsɪləbl/

irreconcilable

Meaning
impossible to make compatible or bring into harmony; unable to resolve differences
Example
The couple separated due to irreconcilable differences.
C2 verb /ɪmˈpjuːn/

impugn

Meaning
to dispute the truth, validity, or honesty of a statement or motive
Example
The lawyer tried to impugn the credibility of the witness.
C2 adjective /ɪnˈsɛnʃənt/

insentient

Meaning
Lacking perception, consciousness, or sensation.
Example
The rock is an insentient object with no awareness.
C2 noun /ˈɪn.fɪ.del/

Infidel

Meaning
a person who does not believe in religion or whose religious beliefs differ from one's own
Example
In historical contexts, crusaders often referred to their enemies as infidels.
C2 noun /ɪmˌpɜːrtərbəˈbɪləti/

imperturbability

Meaning
The quality of being calm and not easily disturbed or upset.
Example
Her imperturbability during the crisis impressed everyone.
C2 adjective /ˌɪm.pəˈfɛk.tə.bəl/

imperfectible

Meaning
Incapable of being made perfect or complete.
Example
Some human qualities are considered imperfectible.
C2 noun /ˌɪnsjəˈlærəti/

insularity

Meaning
The state of being isolated or narrow-minded.
Example
The insularity of the group prevented them from accepting new ideas.
C2 noun /ɪmˈprɑːvɪdəns/

improvidence

Meaning
Lack of foresight or care for future needs.
Example
His improvidence led to financial difficulties.
C2 adjective /ɪmˈplækəbl/

implacable

Meaning
unable to be appeased, calmed, or pacified
Example
The soldiers faced an implacable enemy who refused to surrender.
C2 adjective /ɪˈnɪkwɪtəs/

iniquitous

Meaning
grossly unfair and morally wrong
Example
Slavery was one of the most iniquitous practices in history.
C2 verb /ˈɪməleɪt/

immolate

Meaning
To kill or sacrifice, especially by burning.
Example
The protesters threatened to immolate themselves in front of the building.
C2 noun /ɪnˈtɒksɪkənt/

intoxicant

Meaning
a substance, especially alcohol or a drug, that causes intoxication
Example
Alcohol is the most commonly used intoxicant in many cultures.
C2 noun /ˈaɪdɪl/

idyll

Meaning
An extremely happy, peaceful, or picturesque episode or scene, often idealized.
Example
Their honeymoon was an idyll on a tropical island.
C2 adjective /ˌɪndɪˈvɜːrtɪbəl/

indivertible

Meaning
Not able to be turned aside or diverted.
Example
The judge's indivertible attention was on the case.
C2 noun /ˈɪrɪtənsi/

irritancy

Meaning
the quality or state of causing irritation; legal or formal invalidation
Example
The constant noise became an irritancy to the neighbors.
C2 adjective /ˌɪnkəmˈprɛsəbl/

incompressible

Meaning
Not able to be pressed or squeezed into a smaller space.
Example
Liquids are generally considered incompressible.
C2 noun /ˈɪŋkjʊbəs/

incubus

Meaning
An oppressive or nightmarish burden; in folklore, an evil spirit that lies upon sleeping people.
Example
Debt became an incubus that weighed heavily on the family.
C2 adjective /ˈɪn.dɪ.dʒənt/

indigent

Meaning
lacking the necessities of life due to poverty; very poor
Example
The hospital offers free care to indigent patients.
C2 adjective /ɪnˈseɪʃiət/

insatiate

Meaning
impossible to satisfy; always wanting more
Example
He had an insatiate desire for power.
C2 adjective /ˌɪrɪˈfrændʒɪbəl/

irrefrangible

Meaning
Not to be broken, violated, or infringed
Example
The constitution guarantees certain irrefrangible rights to every citizen.
C2 noun /ɪnˈtɛstəsi/

intestacy

Meaning
The condition of dying without having made a valid will.
Example
His estate was divided according to the laws of intestacy.
C2 noun /ˈɪndələns/

indolence

Meaning
Avoidance of activity or exertion; laziness.
Example
His indolence kept him from finishing the project on time.
C2 noun /ɪˈreɪdiəns/

irradiance

Meaning
the quality of shining brightly; the intensity of light falling on a surface
Example
The irradiance of the sun at noon was overwhelming.
C2 noun /ɪmˈpɒs.tʃər/

Imposture

Meaning
the practice of deceiving others by pretending to be someone else; fraudulent deception
Example
The elaborate imposture lasted for months before investigators uncovered the truth.
C2 noun /ˈɪndɪkənt/

indicant

Meaning
Something that indicates or points to a fact or condition.
Example
High fever is often an indicant of infection.
C2 noun /ˈɪmɪˌteɪtər/

imitator

Meaning
a person who copies the behavior, style, or actions of others
Example
He became famous as an imitator of popular singers.
C2 noun /ɪˌluːmɪˈnɑːti/

illuminati

Meaning
a group of people claiming to have special enlightenment or secret knowledge, often linked to conspiracies
Example
Many conspiracy theories involve the Illuminati as a secret world-controlling group.
C2 noun /ˌɪn.trəˈɡreʃ.ən/

introgression

Meaning
The transfer of genetic information from one species to another through repeated backcrossing.
Example
Introgression plays a significant role in plant breeding.
C2 verb /ˌɪntərˈpoʊz/

interpose

Meaning
To place or insert between one thing and another; to intervene in a situation.
Example
He quickly interposed himself between the fighters to stop the quarrel.
C2 adjective /ˌɪnɪˈsɛnʃəl/

inessential

Meaning
not necessary or important; not essential
Example
The report removes all inessential details to stay focused.
C2 noun /ɪˈluːmɪnənt/

illuminant

Meaning
A substance or device that produces light.
Example
The lamp acted as an illuminant in the dark cave.
C2 noun /ɪnˈvɛstɪtʃər/

investiture

Meaning
the formal ceremony of conferring authority, rank, or office
Example
The king held an investiture ceremony for the new knight.
C2 adjective /ɪnˈsɛnseɪt/

insensate

Meaning
lacking physical sensation or empathy; without feeling or sensitivity
Example
The dictator's insensate cruelty shocked the world.
C2 adjective /ˌɪrɪˈtriːvəbl/

irretrievable

Meaning
impossible to recover or regain
Example
The documents were lost in an irretrievable way after the fire.
C2 noun /ɪnˌtɛl.ɪˈdʒɛntsi.ə/

intelligentsia

Meaning
The group of people engaged in intellectual and cultural pursuits in a society.
Example
The intelligentsia played a key role in shaping public opinion.
C2 noun /ɪnˈhɪərəns/

inherence

Meaning
The state of being an essential or permanent part of something.
Example
The inherence of risk is part of every business venture.
C2 noun /ˌɪnvəˈlɪdɪti/

invalidity

Meaning
the state of being invalid, legally void, or lacking force or validity
Example
The court ruled that the contract suffered from invalidity.
C2 verb ˈɪn.kʌl.keɪt

inculcate

Meaning
Instill an attitude, idea, or habit by persistent instruction; to teach and impress by frequent repetitions.
Example
Parents should inculcate good values in their children.
C2 noun /ˌɪdiəˈsɪŋkrəsi/

idiosyncrasy

Meaning
A distinctive or peculiar characteristic, habit, or mannerism unique to an individual.
Example
One of her idiosyncrasies was humming while working.
C2 verb /ɪmˈpɛnd/

impend

Meaning
to be about to happen; to loom threateningly
Example
Dark clouds suggested that a storm was impeding.
C2 adjective /ɪˈmɪsəbl/

immiscible

Meaning
Describes liquids that do not mix together.
Example
Oil and water are immiscible liquids.
C2 adjective /ˈɪnmoʊst/

inmost

Meaning
Situated at the very heart or deepest part; most private or secret.
Example
She revealed her inmost thoughts to her closest friend.
C2 noun /ˌɪnkænˈdɛsəns/

incandescence

Meaning
the emission of light from a hot object; brilliance
Example
The incandescence of the candle filled the dark room.
C2 adjective /ɪnˈtɛmpərət/

intemperate

Meaning
Lacking self-control; immoderate, especially in indulgence.
Example
The politician's intemperate remarks caused controversy.
C2 verb ɪmˈbaɪb

imbibe

Meaning
Absorb or assimilate ideas or knowledge. To drink or take in.
Example
He tried to imbibe the wisdom from his mentor.
C2 noun /ɪmˈbroʊlioʊ/

imbroglio

Meaning
An extremely confused, complicated, or embarrassing situation.
Example
The political imbroglio lasted for months without resolution.
C2 noun /ˈɪn.tɪ.ɡreɪ.tɪv dɪˈpləʊ.mə.si/

integrative diplomacy

Meaning
A diplomatic approach that seeks to combine or coordinate different elements, interests, or parties into a unified whole.
Example
Integrative diplomacy strengthens alliances.
C2 noun /ˌɪntərˈlɒkjətər/

interlocutor

Meaning
a person who takes part in a conversation or dialogue
Example
She was a skilled interlocutor during the debate.
C2 noun /ɪmˈpjuːɪsəns/

impuissance

Meaning
inability to take effective action; helplessness
Example
The government’s impuissance in the face of crisis frustrated the people.
C2 adjective /ˌɪnɪkˈstɛnsəbl/

inextensible

Meaning
Not capable of being stretched or extended.
Example
This rope is made of an inextensible material.
C2 noun /ˈɪnfəmi/

infamy

Meaning
the state of being well known for some bad quality or deed
Example
His name will live in infamy for the crimes he committed.
C2 verb /ɪˈluːmɪn/

illumine

Meaning
to light up or make clear
Example
The rising sun illumined the mountain peaks.
C2 adjective /ɪˈnɪm.ɪ.tə.bəl/

inimitable

Meaning
So good or unusual as to be impossible to copy; unique.
Example
His inimitable writing style made him a literary legend.
C2 noun /ˈɪd.i.ə.si/

Idiocy

Meaning
extremely stupid behavior or actions; foolishness
Example
The decision to ignore safety protocols was pure idiocy that endangered everyone.
C2 noun /ˌɪntərˈsɛʃən/

intercession

Meaning
the act of intervening or pleading on behalf of another person
Example
The prisoner was released due to the intercession of influential leaders.
C2 adjective /ɪnˈkɒŋɡruənt/

incongruent

Meaning
Not in harmony or keeping with the surroundings or other aspects.
Example
His casual outfit looked incongruent at the formal dinner.
C2 adjective /ˌɪnkoʊˈɜːrsəbl̩/

incoercible

Meaning
Not capable of being forced, compelled, or restrained.
Example
Her incoercible spirit inspired others to fight for justice.
C2 noun /ˈaɪdlər/

idler

Meaning
A person who avoids work or spends time lazily.
Example
He was known as an idler who wasted his days.
C2 adjective /ˈɪkθiɪk/

ichthyic

Meaning
Relating to or resembling fish.
Example
The artist's painting had an ichthyic quality.
C2 verb /ɪnˈsteɪt/

instate

Meaning
To set up in position, office, or authority.
Example
The board decided to instate her as the new director.
C2 adjective /ɪnˈdɛkərəs/

indecorous

Meaning
Not in keeping with good taste and propriety; improper.
Example
His indecorous behavior shocked the audience.
C2 noun /ˌɪn.təˈvɛn.ʃə.nɪ.zəm/

interventionism

Meaning
Government policy of active involvement in economic affairs.
Example
The central bank's interventionism helped stabilize inflation.
C2 noun /ˌɪkθiˈɒlədʒɪst/

ichthyologist

Meaning
A scientist who studies fish.
Example
The ichthyologist discovered a new species of fish in the river.
C2 adjective /ɪnˈkɑːdʒɪtənt/

incogitant

Meaning
Lacking thought; inconsiderate or thoughtless.
Example
It was incogitant of him to make such a rude comment.
C2 verb /ˌɪntərˈsiːd/

intercede

Meaning
to act or plead on behalf of someone in difficulty or trouble; to intervene between parties to help resolve a dispute
Example
She interceded with the teacher to give her brother another chance.
C2 noun /ɪmˈpjuːnɪti/

impunity

Meaning
freedom from punishment or harmful consequences for an action
Example
The dictator ruled with impunity, ignoring international laws.
C2 adjective /ɪnˈɛksərəbl/

inexorable

Meaning
Impossible to stop or prevent; relentless.
Example
The inexorable rise of technology is changing every aspect of life.
C2 noun /ɪnˈdɛntʃər/

indenture

Meaning
A formal legal agreement, contract, or document, often relating to work or service.
Example
In the 18th century, many workers came under indenture to work overseas.
C2 adjective /ɪˈræsɪbl/

irascible

Meaning
Easily angered; quick-tempered.
Example
His irascible nature often got him into trouble.
C2 noun /ɪˈrɛvərəns/

irreverence

Meaning
a lack of respect for people or things that are generally taken seriously
Example
His jokes about religion showed irreverence to the sacred traditions.
C2 adjective /ɪnˈɡreɪʃiˌeɪtɪŋ/

ingratiating

Meaning
Charming or flattering in a way intended to gain favor.
Example
His ingratiating smile made everyone trust him quickly.
C2 adjective /ɪmˈperɪʃəbəl/

Imperishable

Meaning
not subject to decay; lasting forever; eternal
Example
The imperishable beauty of classical art continues to inspire generations.
C2 adjective /ˌɪnkənˈsɪdərəbl/

inconsiderable

Meaning
Too small or unimportant to be worth considering.
Example
The donation was not inconsiderable, though smaller than expected.
C2 noun /ɪkˈsɔːr.ə/

Ixora

Meaning
a tropical flowering shrub with clusters of small bright red, orange, or yellow flowers
Example
The bright red ixora bushes lined the pathway to the tropical garden.
C2 noun /ˈaɪsinəs/

iciness

Meaning
The state of being cold in manner or temperature.
Example
There was a distinct iciness in his voice.
C2 noun /ɪnˈvaɪtər/

invitor

Meaning
a person who invites someone to an event, meeting, or place
Example
The invitor greeted all the guests warmly at the entrance.
C2 verb /ɜːrk/

irk

Meaning
to annoy or irritate someone
Example
It irks me when people are late without calling.
C2 noun /ˈɪnˌɡreɪt/

ingrate

Meaning
An ungrateful person; someone who shows no appreciation.
Example
He worked hard for his friend, but the ingrate never thanked him.
C2 adjective /ɪnˈkoʊətɪv/

inchoative

Meaning
relating to the beginning of something; in an initial stage
Example
The poet used inchoative expressions to depict the dawn of creation.
C2 adjective /ɪnˈklɛmɛnt/

inclement

Meaning
unpleasantly cold, wet, or stormy (usually weather)
Example
The match was canceled due to inclement weather.
C2 adjective /ˈɪnfəntaɪl/

infantile

Meaning
characteristic of an infant; childish
Example
His infantile behavior embarrassed everyone at the meeting.
C2 adjective /ɪnˈsɪpiənt/

incipient

Meaning
in an initial stage; beginning to develop
Example
The incipient storm made the sky turn dark and heavy.
C2 adverb /ˌɪnsəʊˈmʌtʃ/

insomuch

Meaning
to such an extent
Example
He was insomuch tired that he could hardly stand.
C2 noun /ˌɪnəməˈrɑːtə/

inamorata

Meaning
A woman with whom someone is in love; a female lover.
Example
He wrote a poem dedicated to his inamorata.
C2 adjective /ɪmˈpɑːlɪtɪk/

impolitic

Meaning
Unwise or imprudent; not showing good judgment.
Example
It would be impolitic to criticize them openly.
C2 noun /ˌɪmpəˈtjuːəsəti/

impetuosity

Meaning
The quality of being rash or impulsive.
Example
His impetuosity often got him into trouble.
C2 verb /ɪnˈtɜːrpəˌleɪt/

interpolate

Meaning
To insert something, often words, into a text or conversation; to estimate values between known data points.
Example
The editor interpolated a missing paragraph into the manuscript.
C2 noun /ɪmˈpaɪ.ə.ti/

Impiety

Meaning
lack of piety or reverence, especially for a god; disrespect or irreverence toward religious matters
Example
His impiety shocked the religious community when he mocked their sacred ceremonies.
C2 noun /ˌɪntərˈsɛsər/

intercessor

Meaning
a person who intervenes or pleads on behalf of another
Example
He acted as an intercessor to resolve the conflict between the two families.
C2 noun /ˈɪn.dɪ.dʒəns/

indigence

Meaning
a state of extreme poverty or destitution
Example
The charity was founded to help people living in indigence.
C2 adjective /ɪnˈeɪliənəbl/

inalienable

Meaning
Unable to be taken away or transferred; absolute and permanent.
Example
Freedom of speech is an inalienable right.
C2 adjective /ɪˈnɪmɪkəl/

inimical

Meaning
tending to obstruct or harm; unfriendly or hostile
Example
The policy was inimical to the country's economic growth.
C2 noun /ɪmˈpruː.dəns/

Imprudence

Meaning
the quality of being imprudent; lack of care for consequences; recklessness
Example
His financial imprudence led to bankruptcy within just two years.
C2 adjective /ɪnˈæpt/

Inapt

Meaning
not suitable; inappropriate; unfit
Example
His comment was inapt for the formal meeting.
C2 noun /ɪˈmɒdəsti/

immodesty

Meaning
lack of modesty or decency; behavior or attitude that is overly bold or improper
Example
The article was criticized for its tone of immodesty.
C2 adjective /ɪnˈdjuːbɪtəbl/

indubitable

Meaning
Impossible to doubt; unquestionable.
Example
It is indubitable that hard work leads to success.
C2 noun /ˈɪsməs/ or /ˈɪsθməs/

isthmus

Meaning
A narrow strip of land connecting two larger land areas, often with water on either side.
Example
The Panama Canal cuts through the Isthmus of Panama.
C2 noun /ɪnˈfriːkwəns/

infrequence

Meaning
The state of occurring rarely or at long intervals.
Example
The infrequence of their meetings made it hard to maintain close ties.
C2 noun /ˈɪl.i.æd/

iliad

Meaning
An ancient Greek epic poem attributed to Homer, describing the events of the Trojan War.
Example
Students studied the Iliad to understand Greek mythology and heroism.
C2 adjective /ˌɪnæprɪˈhɛnsəbl/

inapprehensible

Meaning
not understandable; incomprehensible
Example
The concept was so abstract that it seemed inapprehensible to most students.
C2 adjective /ˌɪntərˈniːsaɪn/

internecine

Meaning
Relating to conflict within a group, often causing mutual destruction.
Example
The internecine conflict weakened the organization from within.
C2 verb /aɪˈtɪnəˌreɪt/

itinerate

Meaning
To travel from place to place, usually for the purpose of work or preaching.
Example
The missionary itinerated throughout the region to spread the message.
C2 noun /aɪˈkɒnəklæst/

iconoclast

Meaning
A person who attacks or criticizes cherished beliefs, traditions, or institutions.
Example
He was known as an iconoclast who challenged old traditions.
C2 verb /ɪnˈdɛmnɪfaɪ/

indemnify

Meaning
to compensate someone for harm or loss; to secure against future damage or cost
Example
The insurance company agreed to indemnify the client for the damages.
C2 verb /ɪmˈbruː/

imbrue

Meaning
To stain, especially with blood.
Example
His hands were imbrued with blood after the battle.
C2 adjective /ˌɪrɪˈprəʊtʃəbl̩/

irreproachable

Meaning
Beyond criticism; faultless.
Example
The judge was known for his irreproachable integrity.
C2 noun /ɪnˈtɛɡ.jʊ.mənt/

integument

Meaning
A natural outer covering, such as skin, shell, or rind.
Example
The snake sheds its integument as it grows.
C2 adverb /ɪnˈdjuːbɪtəbli/

indubitably

Meaning
Without a doubt; certainly.
Example
She is indubitably the best candidate for the job.
C2 verb /ɪˈmjʊər/

immure

Meaning
to confine or enclose someone against their will
Example
The prisoner was immured in a dark cell.
C2 adjective /ˌɪnkɒntrəˈvɜːtəbl/

incontrovertible

Meaning
impossible to deny or dispute; unquestionable
Example
The evidence was incontrovertible and proved his innocence.
C2 adjective /ˌɪnfɪnɪˈtɛsɪməl/

infinitesimal

Meaning
Extremely small in size or amount; so tiny as to be almost negligible.
Example
The error was so infinitesimal that it didn’t affect the results.
C2 adjective /ˌɪn.ɪˈlʌk.tə.bəl/

ineluctable

Meaning
Impossible to avoid or escape; inevitable.
Example
Death is an ineluctable part of human existence.
C2 noun /ɪnˈɛrənsi/

inerrancy

Meaning
the belief or quality of being free from error; absolute correctness
Example
Some religious groups strongly affirm the inerrancy of their sacred texts.
C2 adjective /ˌaɪsəʊˈθɜːrməl/

isothermal

Meaning
Having or maintaining equal or constant temperature.
Example
The experiment was conducted under isothermal conditions.
C2 adjective /ɪnˈvɪdiəs/

invidious

Meaning
likely to arouse resentment, envy, or anger in others
Example
The manager's invidious remarks created tension among the employees.
C2 adjective /ˌɪndɪˈfætɪɡəbl̩/

indefatigable

Meaning
persisting tirelessly; incapable of being fatigued
Example
Her indefatigable spirit kept the team motivated during the long project.
C2 verb /ɪnˈɡreɪʃiˌeɪt/

ingratiate

Meaning
To gain favor or approval by deliberate effort, often in a flattering way.
Example
She tried to ingratiate herself with her boss by complimenting him often.
C2 adjective ɪnˈdɒm.ɪ.tə.bəl

indomitable

Meaning
Impossible to subdue or defeat.
Example
Her indomitable spirit helped her overcome adversity.
C2 noun /ɪnˌtɜːrpəˈleɪʃən/

interpolation

Meaning
The act of inserting something into a text or estimating values within a sequence of data.
Example
The scientist used interpolation to predict the missing data points.
C2 adjective /ˌɪntərˈlɒkjətəri/

interlocutory

Meaning
Given during the course of a legal action, not final but temporary or provisional.
Example
The judge issued an interlocutory order to maintain the status quo until the hearing.