Definitions play a significant role in argumentation, but that role often appears to be non-argumentative. While there are arguments about definition (in which a proposed definition is the conclusion of the argument) and arguments from definition (in which a stipulated definition is the premise), many uses of definition involve argument by definition, in which a definition is stated or implied as if it were uncontested fact. This sort of argument is used to form associations, make dissociations, exploit ambiguities, and shift the frame of reference. This essay was presented originally as the keynote address at the 10th biennial National Communication Association/American Forensic Association Summer Conference on Argumentation, held in Alta, Utah, in 1997. It is reprinted from the conference volume, Argument in a Time of Change: Definitions, Frameworks, and Critiques (James F. Klumpp, Ed.), pp. 1–11 (Annandale, VA: National Communication Association, 1998). Reprinted by permission of the National Communication Association.