Rhetoric
Rhetoric is the “art of speaking or writing effectively”(Merriam Webster’s Collegiate Dictionary). It derives from the Greek word rhetor, a teacher of oratory.  Aristotle defines it as the art of persuasive speech. 
In ancient Athens, rhetoric flourished because of its democratic assembly.  During the fourth century, teachers, known as sophists, came to Athens from all over Greece to teach this art to the sons of the wealthy. In the following century, Aristotle composed his seminal work on this art.  From it, all subsequent rhetorics have derived.
Today the ability to persuade others is a highly evolved discipline; it calls upon the work of linguists, psychologists, statisticians, and technicians.  Its most powerful expression is advertising.