In the context of Ivan Pavlov's theory of Classical Conditioning, when a conditioned stimulus (CS) is repeatedly presented without the unconditioned stimulus (UCS), the gradual weakening or disappearance of the conditioned response (CR) is called what, and what psychological phenomenon is demonstrated by the subsequent sudden reappearance of the response after some time interval?

According to Ivan Pavlov's theory of classical conditioning, when the pairing of the conditioned stimulus (such as a bell) with the unconditioned stimulus (such as food) is stopped, the conditioned response (such as salivation) gradually decreases and disappears, which is called 'Extinction'. Following this, if the conditioned stimulus is presented again after a rest period, the weakened response may suddenly reappear, which is called 'Spontaneous Recovery'.