in music, an interval whose higher note has a sound-wave frequency of vibration twice that of its lower note. Thus the international standard pitch A above middle C vibrates at 440 hertz (cycles per second); the octave above this A vibrates at 880 hertz, while the octave below it vibrates at 220 hertz.

Because of the close acoustic relationship between two notes an octave apart, the upper A is perceived as qualitatively identical to the lower A, although at a higher pitch. Many musical scales (scale) encompass an octave; in the diatonic scales (major, minor, and modal) of Western music, the octave is an interval of eight notes. It is the only interval to appear as a constant in the musical scales of nearly every culture.

In harmony and in instrumentation, a note paired with its octave is said to be doubled. Melodic doubling in octaves is ubiquitous in all types of instrumental music.