Bit depth
From ColorWiki
The number of bits which are used to store each pixel in an image. Each additional bit adds twice as many color combinations to the number available. 1 bit (bit-mapped - monochrome), 8bit (indexed color and web-safe color), 16 bit (thousands of colors) and 24 bit (true color, millions of colors) are common bit depths.
Increasing the bit depth can greatly affect the quality and size of your images.
See Also
