A digital image is represented by a matrix of values, where each value is a function of the information surrounding the corresponding point in the image. A single element in an image matrix is a picture element, or pixel.
In a color system, a pixel includes information for all color components. In computing it is conventional to use a sampling grid having equal horizontal and vertical sample pitch-square pixels.
The term square refers to the sample pitch; it should not be taken to imply that image information associated with the pixel is distributed uniformly throughout a square region.
Many video systems use sampling grids where the horizontal and vertical sample pitch are not equal.