Pavlov conducted experiments where the tick of a metronome was paired with food. The sound of the metronome came to evoke salivation. Once the ticking sound reliably elicited salivation, Pavlov paired it with the sight of a black square. Following several pairings of the metronome beat with the black square, the sight of the black square evoked salivation. This conditioning is called:

This conditioning is called second-order conditioning.

Explanation:

Second-order conditioning was first demonstrated by Pavlov, with experiments such as the one mentioned in the question. Second-order conditioning occurs when a conditioned stimulus, in this case the tick of the metronome, is paired with a conditioned response, in this case the salivation, and the relationship between the two is strong enough to produce a reliable conditioned response; then the conditioned stimulus can be paired with a second conditioned stimulus and eventually the second conditioned stimulus, in this case the black square, will cause the same conditioned response, the salivation.