Merriam-Webster Collegiate Dictionary - octave

 noun  Etymology: Middle English, from Anglo-French, from Medieval Latin octava, from Latin, feminine of octavus eighth, from octo eight — more at eight  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  3.  a. a musical interval embracing eight diatonic degrees  b. a tone or note at this interval  c. the harmonic combination of two tones an ~ 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 ~ above those corresponding to the digitals  4. the interval between two frequencies (as in an electromagnetic spectrum) having a ratio of 2 to 1  5. a group of eight