A scale series of four sounds, of which the extremes, or first and last, constituted a fourth. These extremes were immutable; the two middle sounds were changeable.

a group of four notes

a four-note portion of a scale. For example, the diminished scale is composed of two tetrachords with identical interval constructions.

A series of four notes that form the intervals of whole-tone, whole-tone, half-tone.

The tetrachord is a concept of music theory borrowed from ancient Greece. The name means four strings, and refers to the Greek lyre. Traditionally, the lowest and highest strings of the lyre were tuned at the interval of a perfect fourth, or a just ratio of 3:4.