| Term | Definition |
| incessant | continuous; without pause |
| inchoate | at an early stage; just beginning |
| incipient | beginning to exist or appear |
| incisive | sharp; keen |
| incite | to urge to action; to stir up |
| inclement | harsh; unfavorable; severe |
| incognito | disguised |
| incoherent | rambling; not logically connected |
| incongruous | unsuited; inappropriate |
| inconsequential | unimportant |
| incontrovertible | certain; undeniable |
| incorrigible | bad beyond correction or reform |
| incredulous | skeptical; disbelieving |
| increment | an increase; a gain |
| incriminate | to charge with a crime; to connect or relate to a wrongdoing |
| incubus | nightmare |
| inculcate | to teach earnestly; to influence someone to accept an idea |
| incumbent | resting or lying down; one who holds a political office |
| incur | to bring upon oneself; to run into some undesirable consequence |
| incursion | a raid; an invasion |