Answer: Conditioned stimulus.
Fear conditioning refers to a learning behaviour in which an aversive stimulus is paired with a neutral stimulus, up to the point in which the neutral stimulus elicits the same response as the aversive one, even when not paired together.
Before the experiences, the doctor was a neutral stimulus because his effect did not depend on previous experience. The aversive stimulus was the shots. The pairing of the doctor with the shots repeatedly elicits the conditional response, which is the crying. It also turns the doctor into a conditioned stimulus.