1) A stimulus that reliably precedes an aversive stimulus and thus may be a conditioned aversive stimulus. Such stimuli may reduce the responding maintained by positive reinforcers, an effect variously called anxiety, conditioned emotional response (CER), or conditioned suppression. In some contexts, the stimulus increases responding, as when presented during avoidance responding or during positively reinforced responding after a history of avoidance; this has been called conditioned acceleration or conditioned facilitation.

2) A stimulus repeatedly preceding an aversive stimulus.