In digital imaging, a pixel is a physical point in an image. It is the smallest controllable element of a picture represented on the screen.
In color images, a color is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow, and black.
Here’s an example of an image zoomed in to show the individual pixels:
A digital image is similar to a grid which stores the pixel values generated by the sensor. Pixels in a digital image have no size until they are displayed on a monitor or printed. For instance, on a 4″ x 6″ print, each pixel in a 5 megapixel image would only measure 0.01mm, while on an 8″ x 10″ print, it will measure 0.05mm.