How Do You Define An Octave?

An octave is a musical term used to denote the interval between two musical notes. The interval can be of eight diatonic degrees between notes that have the same name. The second note will either be double or half the frequency. For instance if the note has a frequency of 200Hz then the octave above it will have 400Hz and one octave higher than that will be 800Hz. It is the second simplest interval in music second only to the unison.

The difference between the notes of different octaves will sound the same to an untrained ear. Therefore the notes which are an octave apart have the same name. Alternatively the term octave is also applied to the set of notes that lie between two notes that are an octave apart.  The word octave originates from an Italian word meaning the number eight.