The special way that the form of a word or phrase differs from the generally accepted form, usually for dialectical reasons.

The special use of a language by speakers from a certain region (dialect) or a social group (sociolect).

Τhe established way to express correctly and acceptably a sentence or a thought. Therefore, a sentence may be correct from a strictly grammatical point of view, but may still sound strange and may not be acceptable because it may violate the “valence” of verb. E.g. the verbs take and receive are more or less synonymous, but in the sentence The students take their books to school, we cannot use receive instead of take.

Τhe expression that does not result from the meanings of the words it consists of. E.g. Tomorrow I’ll hold a dinner party for some friends.

The phrase or clause whose concept does not derive directly from its syntax or its lexical content and is often metaphorical (e.g. It is raining cats and dogs- It is raining really hard).