Classical conditioning, also called "pavlovian conditioning" and "respondent conditioning", is a type of learning involving animals, caused by the association (or pairing) of two stimuli. The simplest form of classical conditioning is reminiscent of what Aristotle would have called the law of contiguity. Essentially, Aristotle said, "When two things commonly occur together, the appearance of one will bring the other to mind." In studies of classical conditioning, the phrase "bring the other to mind" is operationalized so that all people can agree upon the definition.
The most famous example of classical conditioning involved the salivary conditioning of Pavlov's dogs. Pavlov's dogs naturally (unconditionally) salivated to food. Pavlov therefore called the one-to-one correlation between the stimulus (food) and the response (salivation) an unconditional reflex. If a tone (generated by a tuning fork, for example) was reliably sounded for a few seconds before food, however, the tone eventually came to elicit salivation even when the tone was presented alone. Because the one-to-one correlation between the stimulus (tone) and the response (food) involved learning, Pavlov referred to this relationship as a "conditional reflex". The conditional reflex (food-related behavior elicited by a stimulus that has been reliably paired with food) is said to be developed through classical conditioning.
The origins of the two reflexes are different. The food (unconditional stimulus) causing salivation (unconditional response) reflex has its origins in the evolution of the species. The tone (conditional stimulus) causing salivation (conditional response) reflex has its origins in the experience of the individual organism.