1) A class of responses defined by a functional relation with a class of common environmental effects.

2) A class of responses. Responses are assigned to classes because no two can be exactly the same. Special cases include theftee operant, in which the complefion of one response leaves the organism in a position to emit the next, and the discrete or constrained operant (see TRIAL). Classes defined descriptively (descriptive operant) are usually distinguished from those defined functionally (functional operation).

3) An operant is behavior that operates on the environment to produce a change or consequence. Operants are selected in the sense that they increase or decrease as a function of the consequences they produce.

4) A unit of behavior defined by a contingency of reinforcement. Pecking a key is an operant if instances are reinforced in a given situation. A class of responses, all members of which are equally effective in achieving reinforcement under a given set of conditions.

5) A group of responses which produce, and are controlled by, similar consequences. Examples are "eating," "talking," and "thinking about arithmetic." "An operant" is roughly equivalent to the term "a behavior, 11 except that it does not include behavior controlled by a preceding stimulus, such as the behavior in a reflex. "Blinking in response to a loud sound," for example, is not an operant.

6) In the functional usage, an operant is a class modifiable by the consequences of responses in it. It is defined by the relation between consequences and subsequent responding. According to this definition, a response class is not an operant until its modifiability has been demonstrated. In most cases, operants defined descriptively and those defined functionally include roughly the same responses. If they do not, it may be appropriate either to change the method of recording or to search for variables that might limit the modifiability of the class. See also OPERANT BEHAVIOR.

7) In the descriptive usage, usually for the purpose of recording responses, the class is defined in terms of its environmental effect (e.g., a lever-press defined by the operation of a switch; see OPERANDUM). To count as a member of an operant, a response must have a certain force, topography, and so on; another defining property may be the stimuli in the presence of which it occurs (see DISCRIMINATED OPERANT). The effect that defines an operant in this usage may be different from the scheduled consequences of responses (e.g., in a schedule, every response in the class does not necessarily produce a reinforcer).