Bit depth
From The Glossary of Digital Photography
The number of bits used per pixel to represent the color of the pixel. In digital photography, the most common bit depth is 8 bits per channel per pixel. With the three RGB colors (3 x 8 = 24), there are 24 bits for the three channels. Bit depth is also known as true color or millions of colors. Higher-end cameras and scanners use 12 bits per channel, which are represented in 16 bits of data per channel. The extra 4 bits are ignored. See also True Color
