What is Pixel?

Legal Definition
In digital imaging, a pixel, pel, dots, or picture element is a physical point in a raster image, or the smallest addressable element in an all points addressable display device; so it is the smallest controllable element of a picture represented on the screen. The address of a pixel corresponds to its physical coordinates. LCD pixels are manufactured in a two-dimensional grid, and are often represented using dots or squares, but CRT pixels correspond to their timing mechanisms .

Each pixel is a sample of an original image; more samples typically provide more accurate representations of the original. The intensity of each pixel is variable. In color imaging systems, a color is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow, and black.

In some contexts (such as descriptions of camera sensors), the term pixel is used to refer to a single scalar element of a multi-component representation (more precisely called a photosite in the camera sensor context, although the neologism sensel is sometimes used to describe the elements of a digital camera's sensor), while in yet other contexts the term may be used to refer to the set of component intensities for a spatial position, though this is more accurately termed a sample. Drawing a distinction between pixels, photosite and samples avoids confusion when describing color systems that use chroma subsampling or cameras that use Bayer filter to produce color components via upsampling.

The word pixel is based on a contraction of pix (from word "pictures", where it is shortened to "pics", and "cs" in "pics" sounds like "x") and el (for "element"); similar formations with 'el' include the words voxel, texel and maxel (for magnetic pixel).
-- Wikipedia
Legal Definition
The tiniest, square element that can be found on a picture. Depending upon the detail level of a monitor the number of pixels might vary. Usually, pixels are calculated on a PPI (pixel per inch) basis.