In classical conditioning, a formerly neutral stimulus that, after association with an unconditioned stimulus (US), comes to produce a conditioned response. For example, a dog salivates (UR) from the smell of a bone (US) naturally, without any conditioning. Once some neutral stimulus (for example, a "beep" that the dog would not naturally or normally cause the dog to salivate) has been paired with the bone for some time, the dog will salivate (conditioned response or CR) when the "beep" occurs. Once the beep has the capacity to elicit the salivation, it is now considered a conditioned stimulus (CS).
See also: Conditioning