A general online dictionary defines "idiom" as "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."
In English, for example, "raining cats and dogs" for heavy rain, "break a leg" meaning "to wish someone good luck". In ASL, "TRAIN GONE, SORRY", "IN-EYE-OUT-EYE", etc.
Ganezer, Gilda and Posner, Avery (2008). Idioms and Phrases in American Sign Language Volumes 1 through 3, DVDs.
Related posts: a list of idioms in ASL.