Phraseme

A phraseme, also called a set phrase, fixed expression, multiword expression (in computational linguistics), or more specifically idiom, is a multi-word or multi-morphemic utterance whose components include at least one that is selectionally constrained or restricted by linguistic convention such that it is not freely chosen.

In the most extreme cases, there are expressions such as kicked the bucket (to express with flippancy that a person has died) where the unit is selected as a whole to express a meaning that bears little or no relation to the meanings of its parts. Someone unfamiliar with the expression would not know its meaning.

At the other extreme, there are cliches such as in the wrong place at the wrong time, how old are you?, and will you marry me? where meaning is transparent, but the word choices and contexts of use are constrained by the conventions of the English language (hence, *in the bad place at the bad time, *how many years are you?, *do you wish to marry me?).

Both kinds of expression are phrasemes, and can be contrasted with free phrases, expressions where all of the members (barring grammatical elements whose choice is forced by the morphosyntax of the language) are chosen freely, based exclusively on their meaning and the message that the speaker wishes to communicate.