Which term refers to variations in loudness within a piece of music?

The term that refers to variations in loudness within a piece of music is dynamics. Dynamics play a crucial role in expressing the emotional content of a piece, allowing for a range of sounds from soft to loud. This variation can enhance a performance by creating contrast and emphasizing certain sections of the music.

In contrast, tempo pertains to the speed of the beat and how fast the music progresses, which does not concern volume. Melody focuses on the sequence of notes that are perceived as a single entity, emphasizing the tune or main theme of the music, while harmony involves the combination of different musical notes played or sung simultaneously, which contributes to the texture of the music. Dynamics, therefore, stand out as the specific aspect that deals with volume changes, making it the correct answer for identifying variations in loudness.