- Date: 14th century
- 1 : an 8-day period of observances beginning with a festival day
- 2 a : a stanza of eight lines : ottava rima
- b : the first eight lines of an Italian sonnet
- b : a tone or note at this interval
- c : the harmonic combination of two tones an octave apart
- d : the whole series of notes, tones, or digitals comprised within this interval and forming the unit of the modern scale
- e : an organ stop giving tones an octave above those corresponding to the keys
- 4 : the interval between two frequencies (as in an electromagnetic spectrum) having a ratio of 2 to 1
- 5 : a group of eight
In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon which has been referred to as the "basic miracle of music," the use of which is "common in most musical systems." It may be derived from the harmonic series as the interval between the first and second harmonics.
The octave is occasionally referred to as a diapason.
The octave above an indicated note is sometimes abbreviated 8va, and the octave below 8vb. To emphasize that it is one of the perfect intervals, the octave is sometimes designated P8; the other perfect intervals, the unison, perfect fourth, and perfect fifth, are designated PU, P4, and P5.