The term "sensitivity" (as applied to an analytical method's performance) has again become a subject of controversy. Certain authorities (e.g., IUPAC) define a system's sensitivity as the response curve slope (or response/dose), others (e.g., IFCC) in terms of the detection limit. Many investigators have failed to perceive the contradiction between these concepts, wrongly assuming that maximizing "sensitivity" in the first sense maximizes it in the second (i.e., that they are inversely related). The existence of different meanings for this term (when used in the present context) is a source of confusion that has, among other things, led to erroneous ideas relating to immunoassay design. Such confusion should be terminated by adoption of one or the other of the definitions. However, the definitions are not of equal merit. We advance arguments against retention of the "slope" definition, which conflicts with the word's common meaning and is meaningless as an indicator of the performance of a measuring system.