The manner in which something is expressed in words WordNet 3.0 & Open English WordNet

In linguistics, phraseology is the study of set or fixed expressions, such as idioms, phrasal verbs, and other types of multi-word lexical units, in which the component parts of the expression take on a meaning more specific than, or otherwise not predictable from, the sum of their meanings when used independently. Wikipedia

Discipline studying phraeological units Wikidata

The manner in which something is expressed in words. OmegaWiki

Study of set or fixed expressions. Wiktionary