What is the difference between an unconditioned stimulus and a conditioned stimulus in Pavlov’s experiment?

In Ivan Pavlov’s experiments, the distinction between an unconditioned stimulus and a conditioned stimulus lies in whether the response is innate or learned through association.1

• Unconditioned Stimulus: This is a stimulus, such as food, that invariably and automatically elicits a natural, reflexive response—in this case, salivation—without any prior training. It is termed "unconditioned" because the reaction is unconditional.1
• Conditioned Stimulus: This is a previously neutral stimulus, such as the ticking of a metronome, that does not naturally trigger the reflex. It becomes a "conditioned" stimulus only after it is repeatedly paired with the unconditioned stimulus. Through this association, the subject learns to respond to the neutral stimulus alone, creating a conditioned reflex.12

Essentially, while the unconditioned stimulus triggers a biological necessity, the conditioned stimulus relies on the subject's experience and the temporal relationship established during the experiment to elicit the same response.1