1. A speech form or an expression of a given language that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements, as in keep tabs on.
2. The specific grammatical, syntactic, and structural character of a given language.
3. Regional speech or dialect.
4. 1. A specialized vocabulary used by a group of people; jargon: legal idiom.
   2. A style or manner of expression peculiar to a given people: "Also important is the uneasiness I've always felt at cutting myself off from my idiom, the American habits of speech and jest and reaction, all of them entirely different from the local variety" (S.J. Perelman).
5. A style of artistic expression characteristic of a particular individual, school, period, or medium: the idiom of the French impressionists; the punk rock idiom.

[Late Latin idiōma, idiōmat-, from Greek, from idiousthai, to make one's own, from idios, own, personal, private.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.