Final answer:
A neutral stimulus that is paired with an unconditioned stimulus to elicit a response eventually becomes known as the conditioned stimulus (CS). This process is part of classical conditioning, with Pavlov's bell and dog-salivation experiment being a quintessential example.
Step-by-step explanation:
An initially neutral stimulus that comes to elicit a response because of its pairing with a stimulus that causes that response is called the conditioned stimulus (CS). In classical conditioning, this occurs when a neutral stimulus is consistently paired with an unconditioned stimulus (UCS), which naturally elicits an unconditioned response (UCR). Over time, the neutral stimulus itself starts to produce the same response, now termed the conditioned response (CR).
For example, in Ivan Pavlov's famous experiment, a bell (neutral stimulus) was rung just before presenting dogs with meat powder (unconditioned stimulus), which naturally caused them to salivate (unconditioned response). After several pairings, the sound of the bell alone began to elicit salivation (conditioned response), transforming it into a conditioned stimulus.
Thus, a neutral stimulus becomes a conditioned stimulus once it has been paired sufficiently with an unconditioned stimulus during what's known as the acquisition period.