When a person or animal learns to associate a neutral stimulus with a meaningful stimulus, gradually responding to the neutral stimulus in the same way as to the meaningful one, they are exhibiting

Classical conditioning 

Classical conditioning refers to the phenomenon where a biologically important stimulus is paired with a formerly neutral  response to create learning. The most widely cited example of classical conditioning is Ivan Pavlov's study on dogs. In Pavlov's study, a bell (neutral stimulus) was repeatedly paired with food (a meaningful stimulus), leading to a response of salivation. Eventually, the bell itself (neutral stimulus), even in the absence of food (meaningful stimulus) led dogs to salivate (response).