Gamma is the graphical area the abbreviation for gamma correction, a non-standardized method of measurement of contrast for scanners, monitors and photographic materials.
Background of gamma correction is that the human eye won’t perceive a uniform flow from white to black as mathematically linear. In the human perception of contrast is increasing less in the bright areas, while it increases steep in dark areas.
A gamma of 1.0 means no change in contrast. A gamma of 1.8 was historically recommended for Mac monitors, while Windows user has a gamma of 2.2 as the default recommendation. Today a gamma of 2.2 for Mac and Windows is recommended.
Higher gamma values mean more contrast with darker midtones. Lower gamma values represent lower contrast and brighter midtones. Gamma based on the white and black points of an image, which it also does not change.