Bitplane
A way of storing an image as several separate one-bit layers, where the layers together pick out each pixel's colour.
In planar graphics an image is split into several bitplanes, one block of memory per bit of colour depth. A pixel's colour index is set not by one contiguous byte but by the same bit position read across all planes: four planes give four bits and thus 16 possible palette entries. The opposite scheme, "chunky", packs the whole pixel into one byte.
The layout suited the hardware of machines like the Amiga and Atari ST, where the Copper and the blitter could chew through one plane at a time and depth could be added by simply switching on more planes. The drawback was that a single pixel needed writes into several planes, which made fast moving graphics awkward — hence chunky-to-planar routines when the 3D wave arrived. Modes like HAM squeezed in more colours by bending the rules.