The Octatonic Scale is also known as the Diminished Scale. This scale contains eight pitches instead of the seven pitches characteristic of major and minor scales. Constructed of alternating whole steps and half steps, the octatonic scale has a similar quality of directionless looping to the whole-tone scale. Even though the alternation of half steps and whole steps helps to create a sense of location within the scale, the incessant repetition of intervals in the scale makes songs written entirely in one octatonic scale get repetitive rather fast.
There are three octatonic scales, and they’re typically labeled by what’s going on around C. You either have a half step from C to C#, yielding the label OCT[0,1], a half step from C# to D for OCT[1,2], or a whole step from C to D to make OCT[0,2] (in lieu of going to OCT[2,3]).
You can also look at each octatonic scale as being composed of two diminished seventh chords a whole step apart. In one of my college composition professor’s more unusual moments, he said, “Think of the octatonic scales as conjoined twins, connected at the head. But instead of babies, you have diminished seventh chords, and instead of heads, you have a whole step.”
Just like the whole-tone scale is the augmented scale, the octatonic scale is often referred to as the diminished scale because it is the chord scale for diminished chords. (Again, see the articles on Harmony for more.)