A "pixel" is a single physical "light bulb" on a display that is capable of displaying a full color independent of its neighbours. Computer screens displaying their content in pixels. This physical pixel is also called a "device pixel".
In contrast to the device pixel the CSS pixel is a unit of length which roughly corresponds to the width or height of a single dot. The CSS pixel is defined as the physical size of a single pixel at a pixel density of 96 DPI, located an arm's length away from the viewer's eyes. A CSS pixel is therefore equivalent to many device pixels.