Dave has a shoe phobia. it has gotten so bad that he can't go to work because he works construction and his boss will not allow him to come to work barefoot. dave consults with phil the behavior therapist. phil discovers that when dave was young, his dad would beat dave with his wingtips when he did something wrong. for dave, shoes are:

conditioned stimulus

In classical conditioning, a conditioned stimulus is one which is previously a neutral stimulus, which, upon becoming associated with an unconditioned stimulus, eventually triggers a conditioned response. 

An unconditioned stimulus is one which unconditionally, naturally, and automatically elicits or triggers a(n) (unconditioned) response. For example, the smell of food usually triggers hunger.

In contrast, a conditioned stimulus is one which initially does not trigger the same response as the unconditioned stimulus, but because of association, eventually triggers the same response as well. The response to a conditioned stimulus is a conditioned response.

For example, in the famous experiment by Ivan Pavlov, the sound of a bell was paired with the serving of food to dogs. Dogs naturally salivate upon smelling/seeing the food. However, later on, even without the food, when the dogs heard the sound of the bell, they began salivating. The sound of the  bell is the conditioned stimulus, while the salivation of the dog in response to the conditioned stimulus, the bell, is called a conditioned response.