unconditioned stimulus (US)

A stimulus that elicits a response without the necessity of conditioning or learning. For example, in Ivan Pavlov's (1849-1936) experiments, the food was the US because it elicited salivation with no prior conditioning. When a bell was rung before the presentation of food, the bell became the conditioned stimulus (CS) when it began to elicit salivation from the dog even when no food was near.