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Pixels
A pixel begins its life on the camera’s image sensor during that flickering
moment when the shutter is open. The size of each photosite on the image
sensor can be measured, but the pixels themselves are just photons, soon
to be converted into electrical charges, and then into zeros and ones. These
numbers, just like any other numbers that run through your head, have no
physical size.
Although the captured pixels have no physical dimensions, a sensor’s size is
specified just like a digital photo’s, except the count is the number of photosites that it has on its surface instead of pixels. In most cases the numbers
of photosites and the number of pixels are roughly the same since each
photosite captures one pixel. (see Tip box for more on this.) These sizes are
usually specified in one of two ways—by the dimensions or by its total number. For example, an image can be said to have 4368 × 2912 pixels (where
“×” is pronounced “by” as in “4368 by 2912), or to contain 12.7 million pixels
(4368 multiplied by 2912). Since the term “megapixel” is used to indicate 1
million pixels, an image with 12.7 million pixels can also be referred to as a
12.7 megapixel image.

 

Lots to read here>>http://www.photocourse.com/itext/pixels/pixels.pdf