Definitions for "Bit-Depth"
The number of bits used to convey tonality for each pixel. In general, the more bits per pixel the larger the file size. 1-bit or Bi-tonal - a 1-bit pixel has two possible values, black or white; the scanned image has no shading or gray. 8-bit Gray-scale - provides 256 shades of gray ranging from pure white to pure black. 24-bit Color - provides a tonal range of about 16 million different colors.
(or resolution) The amount of colour information available for each pixel in an image, also known as colour resolution. Common pixel depths range from 1 to 24 bits per pixel.
In web design, a measurement of the number of colors based on the number of bits (1s and 0s) allotted by the file or the system. A bit is the smallest unit of information on a computer (one bit can define 2 colors). Strung together, they can represent more values (8 bits can represent 256 values).