| indomitable | Impossible to subdue or defeat | |
| industrious | Diligent and hard-working | |
| ineffable | Too great or extreme to be expressed or described in words | |
| ineluctable | impossible to avoid or evade; inevitable | |
| inestimable | too great to calculate or estimate; immeasurable | |
| inexorable | Impossible to stop or prevent; relentless | |
| infinitesimal | Extremely small or tiny, almost impossible to measure | |
| influx | An arrival or entry of large numbers of people or things | |
| ingenious | Clever, original, and inventive | |
| ingenuous | Showing innocent or child-like simplicity and candidness | |