Pixels are dots used to display an image on a screen or printed matter. The word is a blend of the words picture and element. Usually, the term refers to pixels in a digital context, but it is can be more generalized to refer to any smallest unit of an image.
It is important to recognize that pixels are not a measure of size, though often the term is used as though they are. Many digital cameras, for example, give a measure of pixels-per-inch (PPI), which they embed in the image files created. Users then see this measurement and assume that the units have a real size that uniformly holds true.
A video monitor is made up of many millions of pixels arranged in a grid. Monitors also have a bit depth, which controls how many greys or colors each pixel is capable of displaying. In an 8-bit display, for example, each unit can display 8 bits worth of color, equal to 28 power, or 256, colors. With 24-bit displays, each of the pixels can display in excess of 16 million colors.
While pixels are the smallest complete element of an image, they are comprised of even smaller elements. In a standard RGB monitor, for example, each pixel has three dots within it: a red, blue, and green dot. In theory, these dots all converge at the same point, making them visibly seamless, but in practice some fuzziness is often apparent. The measure of total pixels in an image is referred to as its resolution, and the higher the resolution in a small area, the more difficult it is to distinguish between individual units.
While pixels are often thought of as square, as they usually are in the case of computer monitors, this is not always accurate. Many digital video standards, such as NTSC and PAL, make use of rectangular pixels with an aspect ratio other than 1:1.
The term megapixels is used by digital camera manufacturers to describe the resolution at which the cameras are capable of taking pictures. In most computer contexts, resolution is given as two dimensions, such as in a computer display resolution of 1600 x 1200, which represents a width of 1600 and a height of 1200 pixels. Digital camera manufacturers take the sum total of pixels generated by multiplying the two dimensions, so that a camera capable of producing images at 1600 x 1200 would produce a total of 1,920,000 pixels, or 1.9 megapixels.