【Phrase】 , 【idiom】 ,  【expression】 ,  【locution】  mean a group of words which, taken together, express a notion and may be used as a part of a sentence.

【Phrase】 may apply to a group of words which for one reason or another recurs frequently (as in the language of a people, the writings of an author or school of authors, or the speech of a person or a clique of persons). Sometimes the word means little more than this, but more often it suggests a distinctive character, such as triteness or pithiness or pointedness. In the combinations "noun  【phrase】  " and "verb  【phrase】  " it suggests one of the principal parts of a sentence.

【Idiom】  (see also LANGUAGE 1 ) applies to a combination of word elements which is peculiar to the language in which it occurs either in its grammatical structure or in the meaning which is associated with it but which cannot be derived from it when the elements are interpreted literally; thus, "to keep house," "to catch cold," "to strike a bargain" are examples of idioms.

【Expression】  and  【locution】  are sometimes used in place of  【phrase】 when the idea of a way of expressing oneself is uppermost. Although both terms may be applied to phrases that are generally current, they are perhaps more typically applied to those that are idiosyncratic.

【Expression】 is particularly used when accompanied by a characterizing adjective or clause or 【phrase】 .

【Locution】  is somewhat more bookish than  【expression】 and is therefore often preferred when the reference is to phrases that are peculiar to a language or a group as an 【idiom】 .