What is the term for the word choice used by an author?

The term that best refers to the word choice used by an author is diction. Diction encompasses the specific words and phrases that an author selects to convey their ideas, establish tone, and create imagery, and it significantly impacts the overall style and meaning of the text. Different contexts and purposes can influence the author's diction, leading to variations in formality, complexity, and emotion conveyed through the language.

The other options do hold significance in discussions of language but do not directly refer to an author's specific word choice as succinctly as diction does. Lexicon refers more broadly to the vocabulary of a language or a particular field, language is a general term that denotes communication through words, and vocabulary pertains to the collection of words known and used by a person or within a particular context. Therefore, while related to word use, these terms do not encapsulate the precise concept of diction.