1540s, "a word;" 1580s, "expression of ideas in words," from Late Latin dictionem (nominative dictio) "a saying, expression, word," noun of action from dic-, past participle stem of Latin dicere "speak, tell, say" (source of French dire "to say"), related to dicare "proclaim, dedicate," from PIE root *deik- "to point out" (cf. Sanskrit dic- "point out, show," Greek deiknynai "to prove," Latin digitus "finger," Old High German zeigon, German zeigen "to show," Old English teon "to accuse," tæcan "to teach").
The choice of words. Diction is effective when words are appropriate to an audience. A man might refer to his car as his “wheels” in casual conversation with a friend, but if he were writing an essay for a group of economists, he would write, “People base their decision to buy an automobile on the following considerations,” not “People base their decision to buy wheels on the following considerations.”