The pixel (a word invented from "picture element") is the basic
unit of programmable colour on a computer display or in a computer
image. Think of it as a logical - rather than a physical - unit.
The physical size of a pixel depends on how you've set the
resolution for the display screen. If you've set the display to its
maximum resolution, the physical size of a pixel will equal the
physical size of the dot pitch (let's just call it the dot size) of
the display. If, however, you've set the resolution to something
less than the maximum resolution, a pixel will be larger than the
physical size of the screen's dot (that is, a pixel will use more
than one dot).