A series of shades from white to black. The more shades, or levels, the more realistic an image can be recorded and displayed, especially a scanned photo. Scanners differentiate typically from 16 to 256 gray levels, and high-resolution gray scale is widely used for medical x-rays.
At a resolution of 300 dpi, each square inch comprises 90K pixels (300x300). At one byte per pixel (256 levels), a 10x10" image would take up 9MB of storage (90K x 100). See halftone