Wikipedia: Pixel

An abbreviation for "picture element," a pixel is a single cell in the grid of a bitmapped image. A rectangular collection of pixels is said to be a raster image and is used to encode digital video and to produce computer-generated art.

On most current monitors, a displayed pixel is quite small (about 1/72 to 1/96 of an inch). The resolution? of a display quantifies the number of pixels per inch.

Each pixel in a monochrome image has its own brightness, from 0 for black to the maximum value (e.g. 255 for an eight-bit pixel) for white. In a colour image, each pixel has its own brightness and colour, usually represented as a triple of red, green and blue intensities (see RGB).

Other analogous concepts from computer graphics include voxel? (volume element), texel? (texture element) and surfel? (surface element).