Noun
bitplane (plural bitplanes)
(computing) The set of bits stored at the same relative position in each byte and thus having comparable effects on the overall data, used for sound and image processing, etc.
For example, bitplane data fetches are prioritized over blitter transfers as the immediate display of frame buffer data is considered more important than the processing of memory by the blitter. Source: Internet
There can also be a sixth bitplane, which can be used in three special graphics modes: In Extra-HalfBrite (EHB), if a pixel is set on the sixth bitplane, the brightness of the regular 32 color pixel is halved. Source: Internet
Two special graphics modes were also included: Extra HalfBrite, which used a 6th bitplane as a mask that halved the brightness of any colour seen, and Hold And Modify (HAM), which allowed all 4096 colours on screen at once. Source: Internet