- An interval of frequencies whose ratio of its upper limit to its lower limit is 2 to 1. For instance, a frequency interval from 1,000 to 2,000 Hz.
- An interval of frequencies considered to have eight steps, including the upper and lower limits. Notes one octave apart are perceived by human ears as the same note at different pitches, and not as different notes. The ratio of the upper frequency limit to the lower frequency limit is 2 to 1.