A form of learned association whereby a subject is trained to associate a neutral stimulus with a second stimulus of some significance, either positive or negative.
Example: Ivan Pavlov first demonstrated classical conditioning by ringing a bell (the neutral conditioned stimulus) just before feeding his dogs (food being the unconditioned stimulus). In time, the dogs learned to associate the sound of the bell with food, and would begin to salivate immediately after hearing it.
Grinnell, R. (2016). Classical Conditioning. Psych Central. Retrieved on January 14, 2018, from https://psychcentral.com/encyclopedia/classical-conditioning/