What Foreshortening Means
Seeing an object from an angle is called foreshortening.
Specifically, we define this as, "the compressed appearance of something not perpendicular to us." (right)
Fore means "the front part," indicating that part of the object closest to us. With that in mind, this compressed appearance is achieved by "shortening" those lines which are going from this front part into the distance.
Foreshortening vs. Convergence
Since convergence seems to depend on foreshortening to exist, these two are often treated as part of the same condition. (right)
I have even read that with enough experience, an artist can determine if an object was correctly drawn based on its apparent balance of foreshortening and convergence. Two squares, however, can look quite different depending upon how far away they are from us, with both of them being correct. (left)
Still, under most circumstances there will be a certain amount of foreshortening and convergence based on an object's size, distance and direction to us.
In spite of this, convergence and foreshortening can be independent of each other.
Note in the illustration to the right that no convergence exists within this image of the land, even with foreshortening clearly present. Similarly, a sphere foreshortens all the way around it while nothing on its surface converges toward anything either.
There could also be convergence with no foreshortening, like with a form in motion moving away along a converging path or a row of stationary forms. (left)