Diction (Latin: dictionem (nom. dictio), "a saying, expression, word"), in its original meaning, is a writer's or speaker's distinctive vocabulary choices and style of expression in a poem or story. In its common meaning, it is the distinctiveness of speech: the art of speaking so that each word is clearly heard and understood to its fullest complexity and extremity, and concerns pronunciation and tone, rather than word choice and style. This is more precisely and commonly expressed with the term enunciation or with its synonym, articulation.

Diction has multiple concerns, of which register, the adaptation of style and formality to the social context, is foremost. Literary diction analysis reveals how a passage establishes tone and characterization, e.g. a preponderance of verbs relating physical movement suggests an active character, while a preponderance of verbs relating states of mind portrays an introspective character. Diction also has an impact upon word choice and syntax.

Aristotle, in The Poetics (20), defines the parts of diction (λέξις) as the letter, the syllable, the conjunction, the article, the noun, the verb, the case, and the speech (λόγος), though one commentator remarks that "the text is so confused and some of the words have such a variety of meanings that one cannot always be certain what the Greek says, much less what Aristotle means."