A measure of the average information content per source output unit. Expressed in bits/source output unit (such as bits/pixel). This quantity is maximized when all possible values of the source output are equiprobable. In that case an image consisting of 8 bit pixels will have an average information content of 8 bits/pixel. Coding the source with fewer bits, on average, will result in information loss. If the pixel values are not equiprobable, the entropy is reduced and the image can be losslesly coded with fewer than 8 bits/pixel on average.