Gamma This is a techie term to describe what is called a function. We use this because the correction this performs is not equal to all colors or shades of gray within an image. This function is a curve and not a straight line as the others mentioned above. This term has to do with artificial lighting. Sunlight has a different color temperature than does a regular light bulb.
Florescent lights have yet a different color temperature.
This is why photographs taken indoors with out door film look orange. Because the color temperature from regular incandescent light bulbs is redder than is that generated by the sun. Florescent lights are more green as a rule. Computer monitors also have a color temperature. Humans don't notice this much because they are used to it and they correct for this automatically.