An unconditioned stimulus is a stimulus that does not require any prior learning or conditioning to trigger a response. It was first described by Ivan Pavlov in the context of classical conditioning, a learning process. Pavlov, for instance, in his experiments with dogs, considered food to be an unconditioned stimulus as it was able to trigger salivation reflexively.
The response that it produces is described as an unconditioned response i.e. in this example, the salivation. Unconditioned stimuli are of survival value; typically involves reflexes that sustain the organism, such as acute pain, a loud noise, or even pleasure.
For example, in humans, a common reflexive response might be a startle response to an unexpected loud noise, or a blink response to a stream of air directed at the eyes. Knowing unconditioned stimuli are important for psychologists in probing associative learning and behavior modification, particularly with emotional responses such as fear and pleasure.