Diction, in its original and primary meaning, is the term for a writer or speaker's distinctive choices in vocabulary and style of expression. A secondary, commonly used meaning of diction would be better and more precisely expressed by the word enunciation, the art of speaking with clarity, or in such a way that each word is clearly heard. This secondary sense is concerned with pronunciation and tone, rather than the choice of words or style. A vocabulary is a set of words known to a person or other entity, or that are part of a specific language. ... Enunciation is the act of speaking clearly and concisely. ... Look up pronunciation in Wiktionary, the free dictionary. ...
Diction's concerns with word choice are multiple. Register, or whether words belong to a formal or informal social context, is a large part of it. Analysis of diction in literature reveals much about how a passage establishes tone and characterization. For example, a preponderance of verbs related to physical movement in a story suggests a character who is active while a preponderance of verbs related to states of mind helps to portray a character's introspection. A formality is an established procedure or set of specific behaviors and utterances, conceptually similar to a ritual although typically secular and less involved. ... A colloquialism is an expression not used in formal speech or writing. ... Authors set a tone or mood in literature by conveying an emotion or emotions through words. ...
Etymology
1542, from Late Latin dictionem, from Latin "a saying, expression, word," from dic-, stem of dicere "speak, tell, say," related to dicare "proclaim, dedicate," from Proto-Indo-European base deik- "to point out." The word dictionary is derived from this definition of the word. The dictionary is a list of words with their definitions, a list of characters with their glyphs, or a list of words with corresponding words in other languages. ...
Usage: Style relates both to language and thought; diction, to language only; phraseology, to the mechanical structure of sentences, or the mode in which they are phrased.
The style of Burke was enriched with all the higher graces of composition; his diction was varied and copious; his phraseology, at times, was careless and cumbersome.
Errors in grammar, false construction, a confused disposition of words, or an improper application of them, constitute bad diction; but the niceties, the elegancies, the peculiarities, and the beauties of composition, which mark the genius and talent of the writer, are what is comprehended under the name of style.'' --Crabb.