A multiword expression (MWE), also called phraseme[citation needed], is a lexeme-like unit made up of a sequence of two or more lexemes that has properties that are not predictable from the properties of the individual lexemes or their normal mode of combination. MWEs differ from lexemes in that the latter are required by many sources to have meaning that cannot be derived from the meaning of separate components. While MWEs must have some properties that cannot be derived from the same property of the components, the property in question does not need to be meaning.
For a shorter definition, MWEs can be described as "idiosyncratic interpretations that cross word boundaries (or spaces)".[1]
A multiword expression can be a compound, a fragment of a sentence, or a sentence. The group of lexemes which makup up a MWE can be continuous or discontinuous. It is not always possible to mark a MWE with a part of speech.
A MWE may be more or less frozen.
Example #1 in English: to kick the bucket, which means to die rather than to hit a bucket with one's foot. In this example, that is an endocentric compound, the part of speech may be determined as being a verb. The MWE is frozen, in the sense that no variation is possible.
Example #2 in English: to throw <somebody> to the lions. The pattern <somebody> restricts the usage. The expression is half-frozen because a certain degree of variation is possible but not everything is possible. It is not possible, for instance, to say to the three lions. Like the previous example, the part of speech is a verb.
Example #3 in French: la moutarde <me,te,lui,nous,vous,leur> monte au nez. This MWE is more frozen than the other examples. Let us add that a tense variation is allowed for the verb but we cannot determine what is the part of speech for the whole expression because it is a sentence.