Final answer:
An unconditioned stimulus is a stimulus that naturally elicits a reflexive response without any prior learning, such as the meat powder causing salivation in Pavlov's experiments.
Step-by-step explanation:
An unconditioned stimulus (UCS) is a stimulus that naturally and automatically triggers a reflexive response without the need for prior learning. In the context of classical conditioning, the unconditioned stimulus is something that provokes an unconditioned response (UCR), a natural, unlearned reaction to the stimulus. For example, in Ivan Pavlov's classic experiments, when dogs were presented with meat powder, their natural reaction was to salivate. This salivation is the unconditioned response, and the meat powder is considered the unconditioned stimulus because it inherently elicits that response.