Gamma is a number that represents the relationship between digital pixels and luminance, which is inherently nonlinear. For years, there has been a misconception that gamma had to be corrected due to the inherent deficiency in earlier CRT monitors. In fact, the CRT generated almost the inverse of a human's perception of luminance and thus actually evened out this relationship.
A gamma correction number is applied to ensure that the digital pixels produce a uniform luminance across the entire tonal realm from dark to bright, especially in the midtones. Because LCD monitors were designed to emulate the CRT's gamma, gamma correction is still used. With gamma correction, eight bits per pixel is sufficient; otherwise, 11 bits per pixel would be required. See color depth