operationalism a program in philosophy of science that aims to interpret scientific concepts via experimental procedures and observational outcomes. P. W. Bridgman introduced the terminology when he required that theoretical concepts be identified with the operations used to measure them. Logical positivism’s criteria of cognitive significance incorporated the notion: Bridgman’s operationalism was assimilated to the positivistic requirement that theoretical terms T be explicitly defined via (logically equivalent to) directly observable conditions O. Explicit definitions failed to accommodate alternative measurement procedures for the same concept, and so were replaced by reduction sentences that partially defined individual concepts in observational terms via sentences such as ‘Under observable circumstances C, x is T if and only if O’. Later this was weakened to allow ensembles of theoretical concepts to be partially defined via interpretative systems specifying collective observable effects of the concepts rather than effects peculiar to single concepts. These cognitive significance notions were incorporated into various behaviorisms, although the term ‘operational definition’ is rarely used by scientists in Bridgman’s or the explicit definition senses: intervening variables are theoretical concepts defined via reduction sentences and hypothetical constructs are definable by interpretative systems but not reduction sentences. In scientific contexts observable terms often are called dependent or independent variables. When, as in science, the concepts in theoretical assertions are only partially defined, observational consequences do not exhaust their content, and so observational data underdetermines the truth of such assertions in the sense that more than one theoretical assertion will be compatible with maximal observational data. See also BEHAVIORISM , REDUCTION , RE- DUCTION SENTENCE , THEORETICAL TER. F.S.