What is the effect on the science and practice of behavior analysis when practitioners define terms inconsistently or when those terms are defined in ways that are not useful? Consistent definitions of terms could facilitate the discovery of useful explanations and procedures and allow us to be more effective in helping clients achieve their goals. Are some definitions and terms more useful than others? Panel members will review the history of troublesome terms such as "behavior," "contingency," "consequence," "discriminative stimulus," "frequency," "feedback," "functional," "non-contingent reinforcement," "rate per minute," "relative frequency," "reliability," "social validity," "stimulus," and "trials to criterion," discuss the difficulties caused by these terms and their definitions, and recommend more useful definitions and terms. A lively, but scholarly discussion will likely ensue between panel members and the audience. This session might spark a larger discussion among practitioners about the need for consistent and useful definitions of these troublesome terms.