Rhetoric is at once the facilitation of efficacious discourse as well as the critique of the terms under which discourse comes to be and fails to be efficacious.
I don't know that I would want to be held to that as a formal definition, but it does seem a pretty good point of departure for grappling with the subject. I like that it captures the sense of rhetoric as a constellation of critical thinking and writing skills -- at least genuflecting in the direction of an historical understanding of rhetoric as practical and persuasive arts -- while at once connecting rhetoric explicitly to the archive and practice of contemporary critical theory which is so central to rhetoric as it is taught in the Department at Berkeley at any rate (another, possibly better, way of making this latter point would be to insist on the indebtedness of critical theory to the rhetorical tradition, and then to propose that the Department of Rhetoric at Berkeley is one of the places that properly registers that debt). Whatever its limits, certainly this is a better definition of rhetoric than the one most people seem to hold, even if they keep it to themselves when I'm around. You know, rhetoric as bullshit.