In music theory, which term describes the variation between two pitches?

The term that describes the variation between two pitches is "interval." In music theory, an interval represents the distance between two notes, defined by the number of steps between them in the musical scale. Intervals are fundamental to understanding harmony, melody, and the overall structure of music. They can be classified into different types, such as major, minor, perfect, augmented, and diminished, which help to describe the relationship between the notes more precisely.

A chord consists of three or more notes played simultaneously, rather than the relationship between just two pitches. A scale is a sequence of notes ordered by pitch, which is broader than the concept of an interval. A melody refers to a sequence of musical notes that are perceived as a single entity, which typically involves multiple intervals but is not defined by the relationship of just two notes. This distinction makes interval the correct choice to describe the variation specifically between two pitches.