| Term | Definition |
| ingenuous | innocent |
| inherent | inborn |
| iniquitous | unjust |
| injunction | judicial order |
| innate | inherent, intrinsic |
| innocuous | harmless |
| insidious | sly, treacherous |
| insipid | dull; flat |
| insolvent | bankrupted |
| insular | pertaining to an island |
| intangible | not able to be touched; impalpable |
| interdict | to prohibit |
| interminable | endless |
| interpolate | to insert something additional |
| interregnum | a pause in a continuous series |
| intransigent | uncompromising |
| intrepid | fearless |
| intrinsic | essential |
| intuition | insight |
| invective | denunciation |
| inveigle | to entice |
| investiture | act of bestowing rank or office |
| inveterate | to be firmly established |
| invidious | causing ill will; offensive |
| irascible | easily provoked anger |
| ironic | contrary to what was expected |
| isthmus | a narrow strip of land having water on either side |
| iterate | to utter or do again or repeatedly |
| itinerant | traveling from place to place |
| jargon | meaningless talk; gibberish |
| jetty | a wall built into the water |
| jocose | joking humorous |
| juxtapose | to place side by side |
| ken | range of vision or knowledge |
| labyrinth | maze |
| lachrymal | pertaining to tears |
| laconic | sparing of words; concise |
| lampoon | a strong satire in written form |
| languid | lacking in spirit or interest |
| lascivious | lustful; over-sexed |
| latent | hidden; dominant |
| lethargic | sluggish |
| levity | lack of seriousness |
| lexicon | dictionary |
| levy | to impose a tax or fine |
| licentious | lewd, immoral |
| limpid | clear; as in air or water |
| litany | prayer with response; a prolonged or tedious account |
| lithe | bending easily; flexible |
| litigation | a lawsuit |