The full definition of the word "diction", according to the Merriam-Webster dictionary, is "a choice of words especially with regard to correctness, clearness, or effectiveness". A correct example of using the word diction would be to say, "the politician's diction was so bad, hardly anyone could understand his speech."
Merriam-Webster additionally offers two simple definitions of the word diction: "the way in which words are used in speech or writing" and "the clearness of a person's speech". The origin of the word comes from the Latin root words "dictio", which means "speaking and style", and "dicare", which means "to proclaim." Articulation and enunciation are synonyms for diction.