What Is Gamma?
When voltage is applied to the screen of a monitor, the intensity of the light produced by each pixel is less than that indicated by the applied voltage. That is, one unit of voltage produces less than one unit of light intensity. More precisely, the intensity of light produced by a pixel is equal to the applied voltage - which is always between 0 and 1 - raised to an exponent roughly equal to 2.5. So, if a voltage of 0.4 is applied to a pixel, the resulting light intensity equals 0.4 raised to the 2.5 (0.4 2.5) which equals 0.1011. This exponent - 2.5 in the case of the example - is known as gamma.
To make the brightness and colors of a monitor look right, monitors must be corrected for gamma or, in the language of the industry, gamma corrected.
Note: Different monitors have different gamma values, most somewhere near 2.5.