a word or phrase that is used in familiar, ordinary, and/or informal conversation, often dependent on geographical region, but that is not part of formal language and therefore not used in official or literary contexts;
e.g. Most languages have at least one colloquialism that seems absurd to non-native speakers.
(noun)
a style of communication (written or spoken) that involves the use of familiar and/or informal words, phrasing, and language;
e.g. There can be no colloquialism in an essay that you submit for a grade in a class.