An unconditioned stimulus (US) is a stimulus that naturally triggers a reflexive response without prior learning or conditioning. It is the opposite of a conditioned stimulus, where the response is learned rather than automatic. In classical conditioning, the unconditioned stimulus is paired with a neutral stimulus to eventually produce a conditioned response. Examples of unconditioned stimuli include:

• Tickling (leads to giggling)
• Smell of food (leads to mouth watering)
• Loud noise (leads to jumping)
• Bright light (leads to squinting)
• Pain (leads to wincing)

In Pavlovs experiment, the food was the unconditioned stimulus that automatically triggered the unconditioned response of salivation in dogs. It is important to note that the neutral stimulus becomes the conditioned stimulus after being consistently paired with the unconditioned stimulus.