Define 'octave' in music.

An octave in music is defined as the interval between one musical pitch and another pitch that has double its frequency. This relationship is foundational in music theory and applies across different instruments and voices. When you move up an octave, the higher note vibrates at twice the rate of the lower note, which is why they sound similar yet distinctly different in terms of pitch.

For example, if the lower note is 440 Hz (the standard pitch for the musical note A above middle C), the note one octave higher would be 880 Hz. This doubling of frequency results in a perceivable interval that is recognized universally in Western music.

The other options describe concepts that do not accurately capture the definition of an octave: the first option refers to an interval of a frequency halved, which is not characteristic of how we define octaves; the second option lacks any reference to frequency entirely, focusing on similarity rather than the specific interval concept; and the fourth option describes a technique based on music execution rather than defining a pitch interval. Thus, the response accurately reflects the musical theory underlying octaves.