A multiple of 2. The word should really be something like "bicade" (by analogy with decade) but the "oct" embedded in the stem of the word derives historically to the fact that eight notes correspond to a factor of two in frequency.
Octave
See also
DecadeExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Octave." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Octave.html