A neutral stimulus (defined as any event which does not naturally provoke a response from the test subject) which, under classical conditioning, a subject learns to associate with an unconditioned stimulus.
Example: In Pavlov’s classic dog experiments, the animals learned to associate the sound of a bell (the conditioned stimulus) with the presentation of food (the unconditioned stimulus).
Grinnell, R. (2008). Conditioned Stimulus. Psych Central. Retrieved on May 28, 2015, from http://psychcentral.com/encyclopedia/2008/conditioned-stimulus/