idiom
id.i.om \'id-e--*m\ n [MF & LL; MF idiome, fr. LL idioma individual peculiarity of langua]ge, fr. Gk idio-mat-, idio-ma, fr. idiousthai to appropriate, fr. idios 1a: the language peculiar to a people or to a district, community, or class : DIALECT 1b: the syntactical, grammatical, or structural form peculiar to a language 2: an expression in the usage of a language that is peculiar to itself eith er grammatically (as no, it wasn't me) or in having a meaning that cannot be derived from the conjoined meanings of its elements (as Monday week for "the Monday a week after next Monday") 3: a style or form of artistic expression that is characteristic of an indi vidual, a period or movement, or a medium or instrument
Webster's English Dictionary