What is the term for a writer's or speaker's choice of words?

Diction refers specifically to the choice of words and the style of expression that a writer or speaker uses. It encapsulates aspects such as the connotation of words, the level of formality, and the appropriateness of language for the content and audience. Diction can greatly affect the tone and clarity of a piece, as well as contribute to its overall effectiveness in conveying a message or evoking emotions.

In contrast, syntax deals with the arrangement of words and phrases to create well-formed sentences. Grammar encompasses the set of structural rules governing the composition of clauses, phrases, and words in any given language, focusing on the form rather than the actual choice of words. Vocabulary refers to the range of words known and used by a person or group but does not specifically address how those words are selected or employed in a particular piece of writing.