Answer:
conditioned stimulus (CS)
Explanation:
To understand this better, let's take a look at Pavlov's experiment.
The experiment was simple, Pavlov simply ring a bell before giving his dog a food to see whether it will create some sort of behavioral change. At the beginning of the experiment, ringing the bell before giving the food did not gave any effect to the Dog.
But over-time, the Dog associated the sound of the bell with the food that will come . So eventually, every time the dog hear the sound of the bell, its mouth started to become salivated.
So the effect of the bell has changed for the Dog.
At first, it was irrelevant.
But overtime, The sound the bell becomes trigger a conditioned response after association with the food.
This sound of the bell is what considered to be a conditioned Stimulus.