Definition:
Foreshortening is a technique used in perspective to create the illusion of an object receding into the background.
An example of foreshortening would be when you look down a long staight road lined with trees, the two edges of the road appear to move towards each other and the trees look smaller the further away they are. Or if you're drawing at a person lying on their back with their feet facing towards you, you'd draw their feet larger than their head to capture the illusion.
To see an example of foreshortening, take a look at Monumental Glory by Dana Roach, in the Unexpected Angles Monthly Painting Project.
