| Definition -
(1) The type of speech that is peculiar to a particular place, group, or class.
(2) A group of words that has a meaning different than that suggested by the individual words. For example, saying "I see the light" when you mean to say "I understand."
Etymology - The word derives via Middle French and Late Latin from the Greek idioma, phraseology that is peculiar to oneself (from idios, particular to oneself).
Note: In the 16th and 17th centuries the words idiom and idiotism were synonymous in the above senses. Since then, idiom has superseded idiotism.
Oxford English Dictionary - The word's first citation for sense 1 — which is its first citation in any sense — is from 1575:
"So woulde I wish you to frame all sentences in their mother phrase, and proper Idioma."
(Gascoigne Cert. Notes Instr. Eng. Verse §11 )