Diction: (1) Narrowly defined, a speaker’s (or author’s) word choice. (2) More broadly defined, the general type or character of language used in speech or in a work of literature. (3) With reference to pronunciation and enunciation, the perceived accuracy and clarity with which someone pronounces words.
In the second, broader sense, diction is typically divided into two components: vocabulary and syntax. Vocabulary encompasses the degree of difficulty, complexity, abstractness, formality, and currency of words used, as well as their etymology or origin (native or foreign, Latinate or Germanic, and so forth). Syntax refers to the arrangement — the ordering, grouping, and placement — of words within a phrase, clause, or sentence. The term may also be extended to encompass such things as the complexity or completeness of a given arrangement of words. In critical circles, vocabulary is often described in terms of the “level” of the language used, whereas syntax tends to be discussed in terms of its “texture.”
Poetic diction refers specifically to the choice and phrasing of words suitable for verse.