Foreshortening is the effect that an object is pointing at the viewer, by making it "shorter" than it really is (and usually enlarging the object). Getting this right is one of the "holy grails" of artists.
Technically, all drawings that include the illusion of depth have foreshortening, so only use this tag if the foreshortening is a strong element of the image.
Not to be confused with depth of field, which refers to blurry, out-of-focus portions of an image. An image can contain both, but not necessarily.