Strictly speaking, the smallest fundamental unit of a halftone image, a small point of color, black, or some shade of gray which, when combined with many other dots of varying color and shade, form an image.
Although more technically referred to as spots, in order to distinguish them from halftone dots, computer screen pixels, the smallest portions of light that a scanner can detect, the smallest bits of toner that a laser printer can print, and the smallest areas that an imagesetter or a plotter can expose are also often referred to as dot's.