Cumulative Record. Chapter 28: The Operational Analysis of Psychological Terms. Quote 21

There are three important terms: a stimulus, a response, and a reinforcement supplied by the verbal community. (All of these need more careful definitions than are implied by current usage, but the following argument may be made without digressing for that purpose.) The significant interrelations between these terms may be expressed by saying that the community reinforces the response only when it is emitted in the presence of the stimulus. (p. 419)