Why is the ninth month called September?
c.1300, utaves (plural, via Anglo-French from popular Old French form oitieve, otaves), reformed in early 15c., from Medieval Latin octava, from Latin octava dies "eighth day," fem. of octavus "eighth," from octo (see eight). Originally "period of eight days after a festival," also "eighth day after a festival" (counting both days, by inclusive reckoning, thus if the festival was on a Sunday, the octaves would be the following Sunday). Verse sense of "stanza of eight lines" is from 1580s; musical sense of "note eight diatonic degrees above (or below) a given note" is first recorded 1650s, from Latin octava (pars) "eighth part." Formerly English eighth was used in this sense (mid-15c.)
An interval between musical notes in which the higher note is six whole tones, or twelve half tones, above the lower. From the standpoint of physics, the higher note has twice the frequency of the lower. Notes that are an octave apart, or a whole number of octaves apart, sound in some ways like the same note and have the same letter for their names.
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.