The unconditioned stimulus is something that naturally elicits a response from an organism. The term comes from classical conditioning, a type of psychological procedure first studied systematically by Ivan Pavlov. The unconditioned stimulus is central to a conditioning experiment; it is the wedge used by the researcher to begin intervening in behavior.
Suppose William cries any time someone insults him. According to the theory of classical conditioning, this response could be exploited through the systematic association of insults with some other stimulus. For example, if each time the tormentor insults William, the tormentor also shows him a picture of a unicorn, William may eventually be conditioned to cry upon seeing pictures of unicorns.
In this example, the insult is the unconditioned stimulus. When William cries in response to the insult, this behavior is called the unconditioned response. This behavior relies on a reaction observed to hold true for William before the beginning of the experiment. The new stimulus — a picture of a unicorn — is a conditioned stimulus. If William now cries whenever he sees a picture of a unicorn, this behavior would be called a conditioned response.
The prototypical example of classical conditioning is, of course, Pavlov's experiment with dogs. In the most famous of these experiments, Pavlov would ring a bell before feeding them. The dogs naturally salivated upon being exposed to food; with time, they began to salivate after hearing the bell alone. In this experiment, the food is the unconditioned stimulus because it naturally produces salivation. The bell is the conditioned stimulus; the salivation goes from being an unconditioned to a conditioned response when the dogs exhibit it in response to the bell instead of the food.
Classical conditioning is related to, but distinct from, another psychological procedure called operant conditioning. In operant conditioning, behavior is modified by rewarding or punishing it after it is performed. Giving a dog a treat when it performs a trick upon request is an example of this type of conditioning.
Strictly speaking, there is no unconditioned stimulus in operant conditioning. The reinforcement given to an animal after it performs a behavior, however, is a close analog to the unconditioned stimulus, because it relies on some known pre-existing response. Rewards, like a dog treat, would not be sufficient for operant conditioning if they did not inherently please the dog. Similarly, punishment would not discourage behavior unless it already unconditionally produced pain.