1) A stimulus that acquires its eliciting properties through previous pairing with another stimulus.

2) A stimulus which acquires the property of eliciting a previously unconditioned response is called a conditioned stimulus. A buzzer (conditioned stimulus) which initially has little influence on blood pressure (unconditioned response) comes to elicit changes in blood pressure (conditioned response) when it is paired with an electric shock (unconditioned stimulus). The complete event is called a conditioned reflex.

3) A stimulus which evokes a response or alters some other condition of behavior only because of a history in which it has been paired with a stimulus (often unconditioned) having the same effect.