| Word | Definition |
| Infringe | to violate; to encroach |
| Ingenious | clever; resourceful |
| Ingenuous | naïve and trusting; young; unsophisticated |
| Ingrained | deeply established; firmly rooted |
| Ingrate | ungrateful person |
| Ingratiate | to gain favor with somebody by deliberate efforts |
| Inherent | firmly established by nature or habit |
| Inhibit | to restrain; to retard or prevent |
| Inimical | unfriendly; hostile; harmful; detrimental |
| Inimitable | matchless; not able to be imitated |
| Iniquitous | wicked; immoral; unrighteous |
| Initiate | to begin; to originate; to receive into a group |
| Injurious | harmful |
| Inkling | hint |
| Innate | inborn |
| Innocuous | harmless |
| Innovation | change; introduction of something new |
| Innuendo | hint; insinuation |
| Inopportune | untimely; poorly chosen |
| Inordinate | unrestrained; excessive |
| Inquisitor | questioner |
| Insalubrious | unwholesome; not healthful |
| Insatiable | not easily satisfied; unquenchable; greedy |
| Inscrutable | impenetrable; not readily understood; mysterious |
| Insensate | without feeling |
| Insensible | unconscious; unresponsive |
| Insidious | treacherous; stealthy; sly |
| Insightful | discerning; perceptive |
| Insinuate | to hint; to imply; to creep in |
| Insipid | lacking in flavor; dull |
| Insolence | impudent; disrespectful |
| Insolvent | bankrupt; lacking money to pay |
| Insomnia | wakefulness; inability to sleep |
| Insouciant | indifferent; without concern or care |
| Instigate | to urge; to start; to provoke |
| Insubordination | disobedience; rebelliousness |
| Insubstantial | lacking substance; insignificant; frail |
| Insularity | narrow-mindedness; isolation |
| Insuperable | insurmountable; unbeatable |
| Insurgent | rebellious |
| Insurmountable | overwhelming; unbeatable; insuperable |
| Insurrection | rebellion; uprising |
| Intangible | not able to be perceived by touch; vague |
| Integral | complete; necessary for completeness |
| Integrate | to make whole; combine; make into one unit |
| Integrity | uprightness; wholeness |
| Intellect | higher mental powers |
| Intelligentsia | intellectuals; members of the educated elite (often used derogatorily) |
| Inter | to bury |
| Interdict | to prohibit; forbid |
| Interim | meantime |
| Interloper | intruder |
| Interminable | endless |
| Intermittent | periodic; on and off |
| Internecine | mutually destructive |
| Interpolate | to insert between |
| Interregnum | period between two reigns |
| Interrogate | to question closely; cross-examine |
| Intervene | to come between |
| Intimate | to hint |
| Intimidate | to frighten |
| Intractable | unruly; stubborn; unyielding |
| Intransigence | refusal of any compromise; stubbornness |
| Intrepid | fearless |
| Intrinsic | essential; inherent; built-in |
| Introspective | looking within oneself |
| Introvert | one who is introspective; inclined to think more about oneself |
| Intrude | to trespass; enter as an uninvited person |
| Intuition | immediate insight; power of knowing without reasoning |
| Inundate | to overwhelm; to flood; to submerge |
| Inured | accustomed; hardened |
| Invalidate | to weaken; to destroy |
| Invective | abuse |
| Inveigh | to denounce; to utter censure or invective |
| Inveigle | to lead astray; to wheedle |
| Inverse | opposite |
| Invert | to turn upside down or inside out |
