Note: Many of the entries here will eventually become full-length posts. Some are rough and have not been fully researched. If you have any corrections or would like to add anything, please comment.
Ibid.: abbreviation of Latin ibidem, meaning the same place. It’s used in footnotes, endnotes, and citations to refer to the preceding note.
Iconic: originally meant bearing great symbolic weight or representing its time or place. But now it’s usually just a synonym of famous.
Ideation: the process of forming an idea. It’s not synonymous with idea.
Ignoramus: The plural is ignoramuses. Even if we were speaking Latin, ignorami would not be a logical plural because ignoramus is a verb in that language.
Illegal immigrant: Some consider it offensive—as it’s not the person but the act that’s illegal—but the term is widely used in news publications and elsewhere.
Imaginary vs. imaginative: Imaginary: unreal, made up. Imaginative: creative; of or relating to the imagination.
Imbroglio: a difficult situation or a confused heap. It does not mean a fight or an argument. The plural is imbroglios.
Impetuous: passionately impulsive.
Import vs. importance: Although import does work in place of importance, the longer word is more common.
Imposter vs. impostor: Impostor is more common in American sources. Imposter is more common outside North America. Canadians use them about equally. Other than the spelling, there is no difference between them.
Impractical vs. impracticable: They share much common ground. Impractical means incapable of being put into practice. Impracticable means impossible or unfeasible.
Impromptu: not planned.
Improvise vs. improvize: improvise everywhere.
In case: not incase (though incase is variant spelling of encase).
In fact: not infact.
In point of fact: wordy for in fact.
In situ: in the natural or original position. Latin for in place.
Incentify: can always give way to incentivize (or incentivise outside North America).
Inchoate: in the early stages. Pronounced (in the U.S., at least) in-KO-et.
Independent: not independant.
Infamous: famous for something bad. It does not mean not famous. For that, try unfamous (ignore spell check).
Innit?: British colloquialism for isn’t it?
Inroad: Originally meant an invasion or hostile incursion. Now it usually means (1) a way in, or (2) difficult early progress.
Insinuate: Originally meant to introduce something gradually or by artful means. It’s now usually synonymous with hint. Some careful English-speakers find this newer sense objectionable.
In spite: not inspite.
Inculcate: to impress on the mind or to instill.
Indian, Native American, etc.: Indian is not offensive, as it is self-applied by many Native Americans, but it can be confusing. Using American Indian is one way to get around the confusion. Native American is much more common in writing, but fewer Native Americans self-apply it. In Canada, the terms aboriginal peoples and First Nations are used in addition to the others.
Individual: often just a fancy term for person.
Interrelate (and interrelated, interrelating, etc.): one word, no hyphen.