S-Delta (S∆) refers to a stimulusStimulus is defined as any change in the environment or for... that signals the absence of reinforcement for a particular behavior. When the S-Delta is present, a behavior is less likely to occur because the individual has learned that reinforcement is not available in its presence.
Example
A child asks for candy when a parent is around and usually receives it. However, when the child is with their babysitter (S-Delta), asking for candy does not result in receiving it. The babysitter becomes an S-Delta because the child learns that asking for candy in the babysitter’s presence will not lead to reinforcement (getting candy), reducing the likelihood of asking in that situation.