This is not quite in response to a request by Paul Stob at the Blogora for a "quick" definition of rhetoric for use at family-type gatherings:
When I went to my first Rhetoric Society of America conference in 1998 they were selling a cool T-shirt with the heading "What is rhetoric, anyway?", followed by a bunch of definitions on the back, including Aristotle's "the faculty of discovering in any given case the available means of persuasion", I. A. Richards's "the study of misunderstanding and its remedies", George Campbell's "that art or talent by which discourse is adapted to its end", and Kenneth Burke's "[t]he use of language as a symbolic means of inducing cooperation in beings that by nature respond to symbols", among others. I wore this T-shirt with pride and would point to the back of it whenever someone asked me what I was studying. (To be honest, it also helped me define for myself exactly what it was I was supposed to be studying. And to be honest, I think the definitions helped me a lot more than they helped those asking me what rhetoric is.)
Unfortunately, after many washings, the definitions of rhetoric faded away and in the end, I was left with a T-shirt that only asked, "What is rhetoric, anyway?" Oddly appropriate that all the definitions of rhetoric would self-destruct like that, leaving me again to my own devices.
[Cross-posted as a comment to the Blogora]