rhetoric

Rhetoric is the art of speaking or writing effectively.  It may entail the study of principles and rules of composition formulated by critics of ancient times and can involve the study of writing or speaking as a means of communication or persuasion.

As an art, rhetoric aims to improve the facility of speakers or writers who attempt to inform, persuade, or motivate particular audiences in specific situations.  As a subject of formal study and a productive civic practice, rhetoric has played a central role in the Western tradition.  Its best known definition comes from Aristotle, who considers it a counterpart of both logic and politics, and calls it "the faculty of observing in any given case the available means of persuasion."  Rhetorics typically provide heuristics for understanding, discovering, and developing arguments for particular situations, such as Aristotle's three persuasive audience appeals, logos, pathos, and ethos.  The five canons of rhetoric, which trace the traditional tasks in designing a persuasive speech, were first codified in classical Rome, invention (inventio), arrangement (dispositio), style (elocutio), memory (memoria), and delivery (pronuntiatio).  Along with grammar and logic (or dialectic), rhetoric is one of the three ancient arts of discourse.

From ancient Greece to the late 19th century, rhetoric was a central part of Western education, filling the need to train public speakers and writers to move audiences to action with arguments.  The word is derived from the Greek ῥητορικός (rhētorikós), "oratorical", from ῥήτωρ (rhḗtōr), "public speaker", related to ῥῆμα (rhêma), "that which is said or spoken, word, saying", and ultimately derived from the verb λέγω (loqui), "to speak, say."

Sources:

http://www.merriam-webster.com/dictionary/rhetoric
http://en.wikipedia.org/wiki/Rhetoric