A measure of the brightness and contrast of a monitor or the midtone brightness of digital image. Adjusting the Gamma slider within an image-editing program will adjust the midtones while maintaining detail in the highlight and shadow areas. Adjust the monitor gamma to make the image appear more natural to your eye. A setting of around 18 is right for a Mac monitor while a 2.5 setting is more appropriate for a PC.
A measurement used in sensitometry to describe the angle made between the straight line portion of the characteristic curve of the photograph emulsion and the base of the graph. The gamma is the tangent of the angle so formed.