What is defined as the 'pulse' in music?

The term 'pulse' in music refers specifically to the underlying beat that gives a piece of music its rhythmic foundation. This is the steady, regular heartbeat that listeners can feel, often guiding their sense of timing and movement within the music. The pulse is crucial as it provides a framework for both performers and listeners, making it easier to follow the music and participate in activities like dancing or clapping along.

In contrast, the emotional impact of a piece relates more to its expressive qualities and how it affects the listener's feelings, rather than the foundational rhythm itself. Variation in tempo refers to changes in the speed of the music, which can enhance the emotional expression or dynamics but does not constitute the pulse. Lastly, the harmonic structure involves the chords and their progression, which also plays a significant role in music but does not define the pulse. Therefore, the correct answer directly identifies the essential rhythmic element that serves as the music's consistent beat.