In classical conditioning, it is the previously neutral stimulus that in simultaneous presentation with the unconditioned stimulus eventually triggered a behavioural (often reflexive) response to an organism under observation.
Ivan Pavlov was the first to demonstrate classical conditioning on dogs. He was able to show that using a neutral stimulus such as a bell sound he paired with food, he eventually triggered salivation in the dog every time the dog hears the bell sound. Thus, even with or without food, the dog salivated every time the dog heard the bell sound. This conditioned response made the bell sound a conditioned stimulus.
See also: neutral stimulus