The answer is: conditioned stimulus (CS)
Conditioned stimulus refers to the neutral stimulus that trigger a specific response after being associated for a prolonged period of time with unconditioned stimulus.
In the case above:
- the toilet flush is previously a neutral stimulus that does not trigger any response.
- The toilet flush later on paired with unconditioned stimulus (hot water) that make jeff scream in pain
- Because of this, the toilet flush become a conditioned stimulus. This make jeff associate the toilet flush with hot water and make him twitch whenever he hear toilet flush while bathing.