An octave on the piano starting from C.
An octave is the interval between any musical note and its most perfect concord, which is double its pitch, and occupies the position of the eighth note from it on the diatonic scale: C-c, E-e, A-a, etc. The name is given to the eighth note itself as well as to the interval.
The frequency of a note in music is twice that of the one an octave below it. On a keyboard instrument the frequencies of a note and its successive octaves are in the proportions 1:2:4:8:16, etc. This is a logarithmic scale to the base 2.
The octave is the simplest interval, occurring at the first overtone; the two notes sound remarkably similar, differing only in register and blending almost into a single note.
20Hz–40Hz is often considered the bottom octave. Each octave added on the bottom requires that speakers move four times as much air.
Although the term 'octave' is generally taken to mean a perfect octave comsisting of six whole steps, there is also a diminished octave which is one half-step less and a augmented octave is one step more.