Gamma
The curve that relates the numbers stored in an image to the amount of light shown on screen.
Also: tone curve, transfer function
The numbers in an image file do not rise in step with light output. A value of 128, halfway to 255, produces only about a fifth of the light of full white on a typical screen. This curve is called gamma. It exists because the eye is far more sensitive to differences between dark tones than between light ones, so the curve spends more of the available numbers on the darks, where they are needed. A wrong gamma setting makes pictures look washed out or too dark and contrasty.