| Term | Definition |
| inopportune | Unsuitable or inconvenient, especially as to time. |
| inquire | To ask information about. |
| inquisition | A court or tribunal for examination and punishment of heretics. |
| inquisitive | Given to questioning, especially out of curiosity. |
| inquisitor | One who makes an investigation. |
| inroad | Forcible encroachment or trespass. |
| insatiable | That desires or craves immoderately or unappeasably. |
| inscribe | To enter in a book, or on a list, roll, or document, by writing. |
| inscrutable | Impenetrably mysterious or profound. |
| insecure | Not assured of safety. |
| insensible | Imperceptible. |
| insentient | Lacking the power of feeling or perceiving. |
| inseparable | That can not be separated. |
| insidious | Working ill by slow and stealthy means. |
| insight | Intellectual discernment. |
| insignificance | Lack of import or of importance. |
| insignificant | Without importance, force, or influence. |
| insinuate | To imply. |
| insipid | Tasteless. |
| insistence | Urgency. |
| insistent | Urgent. |
| insolence | Pride or haughtiness exhibited in contemptuous and overbearing treatment of others. |
| insolent | Impudent. |
| insomnia | Sleeplessness. |
| inspector | An official appointed to examine or oversee any matter of public interest or importance. |
| instance | A single occurrence or happening of a given kind. |
| instant | A very brief portion of time. |
| instantaneous | Done without perceptible lapse of time. |
| instigate | To provoke. |
| instigator | One who incites to evil. |
| instill | To infuse. |
| instructive | Conveying knowledge. |
| insufficiency | Inadequacy. |
| insufficient | Inadequate for some need, purpose, or use. |
| insular | Pertaining to an island. |
| insulate | To place in a detached state or situation. |
| insuperable | Invincible. |
| insuppressible | Incapable of being concealed. |
| insurgence | Uprising. |
| insurgent | One who takes part in forcible opposition to the constituted authorities of a place. |
| insurrection | The state of being in active resistance to authority. |
| intangible | Not perceptible to the touch. |
| integrity | Uprightness of character and soundness of moral principle. |
| intellect | The faculty of perception or thought. |
| intellectual | Characterized by intelligence. |
| intelligence | Capacity to know or understand. |
| intelligible | Comprehensible. |
| intemperance | Immoderate action or indulgence, as of the appetites. |
| intension | The act of stringing or stretching, or state of being strained. |
| intensive | Adding emphasis or force. |