noun, plural: neutral stimuli
In classical conditioning, it is the type of stimulus that initially or normally does not elicit an overt behavioural response (apart from focusing attention) in the observed organism but when paired with the unconditioned stimulus and presented simultaneously to the organism, the organism eventually responds to it (neutral stimulus).
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 caused the dog to salivate every time the dog hears the bell sound.
See also: classical conditioning, conditioned stimulus