What are the conditioned stimulus and unconditioned stimulus in Pavlov’s dog experiment?

In Ivan Pavlov’s experiments on classical conditioning, the stimuli are defined by their ability to elicit a specific physiological response:

• Unconditioned Stimulus (US): This is the food (such as dry meat powder), which invariably and automatically elicits salivation in the dog without any prior training.12
• Conditioned Stimulus (CS): This is an arbitrary signal, such as the ticking of a metronome or the sound of a bell, that precedes the delivery of food. Initially, this stimulus does not cause salivation, but after repeated pairings with the unconditioned stimulus, it becomes capable of eliciting a conditioned response on its own.12

The dog’s natural salivation in response to the food is termed the unconditioned response, while the salivation that occurs in reaction to the conditioned stimulus alone is known as the conditioned response.12