/ ˈɒktɪv; NAmE ˈɑːk-/ noun
( music ) the difference (called the interval ) between the first and last notes in a series of eight notes on a scale :
to play an octave higher
Orbison's vocal range spanned three octaves.
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WORD ORIGIN
Middle English (originally referring to a period of eight days beginning with the day of a Church festival): via Old French from Latin octava dies eighth day.