1) Any thing or event. Note that a stimulus exists even if nobody responds to it. What or how much is called 11 a stimulus" is largely arbitrary. one letter of the alphabet can be "a stimulus," or one can talk about a whole book as "a stimulus." The plural of stimulus is stimuli.

2) Any physical event, combination of events, or relation among events. The stimulus vocabulary classifies as-pects of the environment in much the same way that the response vocabulary classifies aspects of behavior. Like responses, stimuli may be described in terms of physical or behavioral properties and, again like responses, they may be defined in terms of descriptive or functional classes . The term may refer to any of the following: specific instances of phys-ical events (e.g., the sound of a bell); combina-tions of events, sometimes also referred to as compound stimuli or stimulus complexes (e.g., feeder operations, with accompanying auditory and visual components); the absence of events (e.g., a dark chamber as a stimulus); a relation among events (e.g., the matching rela-tion as a stimulus in a matching-to-sample problem); specific physical properties of events (e.g., green referred to as a stimulus even though it is only one of several properties of a light); classes defined by physical properties (e.g., a stimulus class consisting of all lights length); and classes defined in terms of behav-ior within certain limits of intensity and behavioral functions (e.g., classes of effective discriminative stimuli or of stimuli effective as reinforcers or punishers). --- When stimulus is used descriptively, the continua or dimensions along which stimuli vary (e.g., intensity, wavelength or frequency, spa-tial extent, duration) may be discussed in at least two distinct ways: A change in some stim-ulus property is said to produce a change in the stimulus, or it is said to change one stimulus to another. The usage is typically determined by convenience of exposition rather than conven-tion (e.g., the light was changed from green to blue is equivalent to the green light was replaced by blue). When stimulus is used functionally, an event is not a stimulus unless it exerts control over behavior. Functional classes can often be characterized verbally even though their limits cannot be specified adequately in physical terms (e.g., red stimuli do not necessarily include wave lengths in the red region of the spec-trum).

3) A stimulus is any physical event or condition, including the organism's own behavior. A stimulus may have many different functional relations to an organism's repertoire. It may be an eliciting stimulus for a response in a reflex. It may serve as a discriminative stimulus which precedes an operant performance. It may be a conditioned reinforcer or a primary reinforcer which follows an operant performance and increases its frequency. It may be an aversive stimulus whose termination increases the frequency of an operant performance. Finally, it may have no demonstrable effect on the organism's repertoire.

4) Any physical event or condition in the experimental situation including the organism's own behavior. Not to be confined, as it sometimes is, to those events or conditions which have a demonstrable effect on the organism. See particular names for different types of stimuli.

5) Any physical object or event in the environment in which a person is behaving.

6) A single instance of a class of stimuli.

7) A physical object or event that has an effect on the behavior of an individual. Stimuli may be internal (e.g., pressure, pain, covert statements) or external to the person. Stimuli that frequently are arranged in behavior analysis programs include reinforcing stimuli, aversive stimuli, and discriminative stimuli.