A new set of terminology grew with Pavlovian experiment. Before conditioning, the bell and the food are unrelated stimuli. Before conditioning, the bell is considered as a neutral stimulus, because it has no effect on the response of interest (salivation). It only generates certain orienting responses on the part of the dog, like pricking of ears and making minor body movements, Food is considered an Unconditioned Stimulus (UCS), because it naturally evokes salivation, which is an unlearned behavior. The salivation caused by food is not associated with any previous learning. Hence, salivation in response to food is called an Unconditioned Response (UCR).
During conditioning, the bell is sounded just before the presentation of food. The goal of conditioning is that the bell should get associated with food and therefore, should bring about the same kind of response (salivation) as food does. During this phase, the amount of salivation gradually increases each time the bell is sounded. Finally, when conditioning is complete, the bell alone is capable of evoking salivation. The bell, which was initially a neutral stimulus, has now acquired the status of a Conditioned Stimulus (CS). At this time, salivation in response to bell, the conditioned stimulus, is considered the Conditioned Response (CR). Salivation for the bell is a learned response. After conditioning, the CS evokes the CR.