| Term | Definition |
| inevitable | unavoidable; sure to happen |
| inexorable | unyielding |
| infallible | certain; without mistakes |
| infamous | having an extremely bad reputation; destable |
| infantile | childish; immature |
| infectious | passing on a disease with germs; likely to spread; contagious |
| infer | to conclude; to derive by reasoning |
| infernal | hellish; fiendish; diabolical |
| infidel | unbeliever |
| infinitesimal | exceedingly small; minute |
| infirmity | weakness; feebleness |
| inflated | puffed up; swollen |
| influx | a flowing in |
| infraction | the breaking of a law or rule |
| infringe | to break a law; to violate; to trespass |
| ingenious | clever |
| ingenuous | simple; innocent; naive |
| ingrate | ungrateful person |
| ingratiate | to work one's way into another's favor |
| inherent | inborn |