Each pixel in a digital image has a certain level of brightness ranging from black (0) to white (1). These pixel values serve as the input for your computer monitor. Due to technical limitations, CRT monitors output these values in a nonlinear way:

Output = Input ^ Gamma

When unadjusted, most CRT monitors have a "gamma" of 2.5 which means that pixels with a brightness of 0.5, will be displayed with a brightness of only 0.18 (0.5^2.5) in non-colormanaged applications (*). LCDs, in particular those on notebooks, tend to have rather irregularly shaped output curves. Calibration via software and/or hardware ensures that the monitor outputs the image based on a predetermined gamma curve, typically 2.2 for Windows, which is approximately the inverse of the response of the human vision. The sRGB and Adobe RGB color spaces are also based on a gamma of 2.2.

A monitor with a gamma equal to 1.0 would respond in a linear way (Output = Input) and images created on a system with a gamma of 2.2 would appear "flat" and overly bright in non-color managed applications (*).