In classical conditioning, is an originally irrelevant stimulus that, after association with an unconditioned stimulus, comes to trigger a conditioned response.

conditioned stimulus

Explanation:

In Pavlov’s classical conditioning experiment with dogs, he used a bell (neural stimulus) which initially did not generate salivation until after pairing it with the meat powder (unconditioned stimulus).  The meat powder (unconditioned stimulus) naturally triggers salivation (unconditioned response).  After the association of the bell with the meat powder, salivation as a conditioned response became triggered  by the tone of the bell alone.

The tone of the bell, which was originally irrelevant, is now the conditioned stimulus that now triggers salivation (conditioned response).

i thinking chemical change