US - A stimulus that causes an Unconditioned Response on the part of an organism without it needing to be taught (i.e., without it needing to be “Conditioned”).

In Pavlov's dog experiments, when the dog was presented with the site and smell of food, it salivated. In this case, the site and smell of food is the Unconditioned Stimulus, because it evokes salivation (the Unconditioned Response) in the dog naturally, without the dog having to be trained (i.e., "Conditioned").