oc·tave / ˈäktəv; ˈäkˌtāv/ • n. 1. Mus. a series of eight notes occupying the interval between (and including) two notes, one having twice or half the frequency of vibration of the other. ∎  the interval between these two notes. ∎  each of the two notes at the extremes of this interval. ∎  these two notes sounding together.2. a poem or stanza of eight lines; an octet.3. the eighth day after a church festival, inclusive of the day of the festival. ∎  a period of eight days beginning with the day of such a festival.4. Fencing the last of eight standard parrying positions.

octave (eccl.) formerly pl., eighth day after a festival, period of eight days beginning with the festival XIV; (pros.) group of eight lines of verse XVI; (mus.) note eight diatonic degrees above a given note XVII; interval, or series of notes. between a note and its octave; (fencing) XVIII; group of eight XIX. — (O)F. octave, superseding semi-pop. oitieve, utave — L. octāva, fem. of octāvus eighth, f. octō EIGHT.
So octavo size of the page of a book for which the sheets are so folded that each leaf is one-eighth of a full sheet XVII, earlier in o. XVI ‘in an eighth’; abl. of L. octāvus. octet(te) (mus.) composition for eight instruments or voices; (pros.) group of eight lines. XIX. — It. ottetto, or its deriv. G. oktett; f. otto eight.

octave. Interval of 8 notes, counting bottom and top notes. Notes an octave apart have same letter-names. Interval from, say, D to next D above is perfect octave; from D up to D♭ and from D up to D♯ are diminished and augmented octaves respectively. Also double octave, 2 octaves; at the octave, to be perf. octave higher than written; in octaves, to be perf. with each note doubled one or more octaves above or below.

octave In music, the interval between any given note and another one that is exactly twice (or half) the frequency of the first and thus, acoustically, a perfect consonance. In Western music, it encompasses the eight notes of the diatonic scale.

octave A cask for wine containing one‐eighth of a pipe, about 13 imperial gallons (59 L).