Gamma
A curve that bends the middle tones lighter or darker while black and white stay put.
See it in Dither engine. Under Image processing, move Gamma and watch the middle greys shift.
In plain words
Eyes notice small changes in dark tones far more than in bright ones. So pictures are stored with a curve that spends more numbers on the darks. That curve is gamma. A gamma setting in a tool bends the middle tones: lower lifts them, higher sinks them. Pure black and pure white never move.