Final answer:
A conditioned stimulus is a previously neutral stimulus that, after being paired with an unconditioned stimulus, elicits a conditioned response.
Step-by-step explanation:
A conditioned stimulus (CS) in the realm of classical conditioning is a previously neutral stimulus that, after being paired with an unconditioned stimulus (UCS), comes to elicit a conditioned response (CR). For example, in Ivan Pavlov's famous experiments, the sound of a bell became a conditioned stimulus when it was repeatedly paired with the presentation of food, an unconditioned stimulus, to dogs. Eventually, the dogs learned to associate the ringing of the bell with food and began salivating upon hearing the bell, even in the absence of food. This salivation in response to the bell is the conditioned response. Over time, if the bell is rung without the subsequent presentation of food, this association can weaken in a process called extinction, leading to a decrease or disappearance of the conditioned response.