Question

What is an idiom?

Answer 1

id·i·om Noun /ˈidēəm/

Synonyms:
noun: dialect, speech, locution, language, phrase

idioms plural
A group of words established by usage as having a meaning not deducible from those of the individual words (e.g., rain cats and dogs, see the light)
A form of expression natural to a language, person, or group of people
he had a feeling for phrase and idiom
The dialect of a people or part of a country
A characteristic mode of expression in music or art
they were both working in a neo-Impressionist idiom

Answer 2

Good link for idioms
http://www.usingenglish.com/reference/idioms/