what is an unconditioned stimulus

An unconditioned stimulus (UCS) is a stimulus that naturally triggers a reflexive response without prior learning or conditioning. For example, food is an unconditioned stimulus that causes salivation in dogs, which is an unconditioned response. Other examples of unconditioned stimuli include smells, pain, temperature, water, and sexual stimulation. In classical conditioning, the unconditioned stimulus is paired with a neutral stimulus, which eventually becomes a conditioned stimulus that can trigger a similar response.