An octatonic scale is any 8-note scale.
The octatonic scale may also refer to Diminished, the 4L 4s Multi-MOS scale of the diminished temperament. In 12edo, this is the scale of alternating half and whole steps (1 2 1 2 1 2 1 2), but it can also be realized in other tunings such as 44edo (3 8 3 8 3 8 3 8). It is used in both classical and jazz music; in jazz theory, it is usually called the diminished scale. Due to the even number of tones they have, the octatonic scale is better served by reversing the roles of fifths and sixths, creating many regular triads.