Rhetoric, which has been since classical times, has been the study of the persuasive and expressive resources of language: the techniques of language and thought that can be used to construct effective discourses. Aristotle separated rhetoric form poetics, treating rhetoric as the art of persuasion and poetics as the art of imitation or representation.
Culler, Jonathan. Literary Theory: A Very Short Introduction. Oxford U Press: New York, 1997.