| inscrutable | Impossible to understand or interpret | |
| inordinate | Exceeding reasonable limits; immoderate | |
| innate | Inborn; natural | |
| inimitable | So good or unusual as to be impossible to copy; unique. | |
| ingratiate | Gain favor with someone by deliberate efforts | |
| ingenuous | Showing innocent or child-like simplicity and candidness | |
| ingenious | Clever, original, and inventive | |
| influx | An arrival or entry of large numbers of people or things | |
| infinitesimal | Extremely small or tiny, almost impossible to measure | |
| inexorable | Impossible to stop or prevent; relentless | |