Image histogram is a type of histogram that represent the lightness distribution in digital image. In order to clarify the idea, I am going to give an example.

Suppose we have an image, say depth is 2 bits (2 bpp). Therefore, the possibility value 0 – 3.

As I mentioned, histogram is going to represent the distribution of pixel value. You can see, there are 5 pixels with value 0, 7 pixels with value 1, 9 pixels with value 2 and 4 pixels with value 3. These information is tabulated as follows.

As you know, like histogram in math, histogram is presented using graph. The following graph represent the distribution in image above.