Average contrast = the average intensity within the object relative to its surroundings = Saliency / Area
Suppose the curve on the left is the gray scale function of the image. Then the tip marked with a horizontal line corresponds to an object. Finally, the average height of the red lines is the average contrast of the object.
The advantage of average contrast is that it is more 'robust' than Contrast. Adding a single white pixel to a light object may dramatically increase its contrast, but not its average contrast.
For other measurements see Measuring objects.
These parameters are shown in the output of Pixcavator under "Contrast".
To see how this can be applied, browse our numerous image analysis examples.