In classical conditioning, which term describes a learned decrease in the conditioned response?

The term is known as extinction.

Explanation:

In classical conditioning one acquires a desired behavior by stimulus and the responses associated to these.

The term extinction refers to the reduction and eventual annulation of a certain behavior previously learnt, either by the omission of the previous conditioned stimulus or the reinforcement that comes after the desired behavior.

Extinction is the term used to refer when a learned behaviour ceases to be performed. This will happen if the conditioned stimulus occurs repeatedly without being paired with the unconditioned stimulus.