Definition and background:
An interval spanning seven diatonic degrees, eleven semitones. The frequency of a note one octave above another will have exactly twice as many Hertz as the frequency of the note an octave below it. to modern ears, the "same" pitch separated by register. The octave is the eighth note counting by step up a diatonic scale and in modern notation would be called by the same letter name as the starting pitch. Medieval listeners, however, did not consider octaves to be equivalent, so, for instance, a cadence on the upper octave could not be substituted for a lower note. See interval. an interval eight diatonic scale degrees above it. two notes an octave apart have the same letter name, and form the most consonant interval possible
A succession of eight notes comprising a scale, the eighth one having twice as many vibrations per second as the first.
A difference of pitch where one tone has a frequency that is double or one-half of the frequency of another tone.
For more information on the music term "Octave" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary