Classical conditioning if often described as a learning process that occurs in a series of three phases or stages — before conditioning, during conditioning and after conditioning. Five key terms are used to explain the entire process: the unconditioned stimulus, the unconditioned response, the neutral stimulus that becomes a conditioned stimulus, and the conditioned response. 

The unconditioned stimulus (UCS) is any stimulus that consistently produces a particular, naturally occurring, automatic response. 

The unconditioned response (UCR) is the response that occurs automatically when the UCS is presented. A UCR is a reflexive involuntary response that is predictably caused by a UCS. 

The neutral stimulus (NS) is any stimulus that does not normally produce a predictable response. In particular, this stimulus is ‘neutral’ to the UCR. 

Therefore, the conditioned stimulus (CS) is the stimulus that is ‘neutral’ at the start of the conditioning process but eventually triggers a very similar response to that caused by the UCS — a response that has become a conditioned response. 

The conditioned response (CR) is the learned response that is produced by the CS. The CR occurs after the NS has been associated with the UCS and has become a CS. The behaviour involved in a CR is very similar to that of the UCR, but it is triggered by the CS alone.