A non-standardized measure of contrast for color scanners, video monitors and photographic materials.
A gamma of 1.0 usually means normal or no change in contrast. Historically, Gamma 1.8 was typically recommended for Macintosh monitors. Windows monitors use gamma 2.2 as their recommended standard. For most modern uses, and particularly where cross platform uniformity is desired, gamma 2.2 is recommended for Mac and Windows.
Higher gamma numbers usually mean more contrast with darker mid-tones. Lower gamma numbers usually mean lower contrast and lighter mid-tones. Gamma pivots on (does not change) the white and black point of an image. Gamma is typically applied by raising fractional L*, fractional RGB or fractional XYZ to the power of gamma.