A pixel is a basic unit of composition for an image to be displayed on currenet video display devices..
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TV Short for picture element. Not to be confused with Pixar, one of the most well known early computer graphics companies.
Or Voxel, which is volume element.
A pixel is NOT a unit of measurement. (A unit of measurement is something that has a precisely defined length. But pixels vary in size - and can be any size you want.)
A pixel is an area that's painted a single colour. It's typically needle point-sized - and occurs along with other pixels in a rectangular grid - such as pixels on a computer monitor, graphic image or paper print-out.