A weapon with a telescoping bolt (also known as an overhung bolt) is one with a bolt which telescopes over, that is, wraps around and past, the breech end of the barrel. This feature reduces the required length of a weapon such as a submachine gun significantly, and it allows rifle designs to be balanced around the pistol grip in a way that gives "pointability" similar to a pistol's.
An overhung bolt is also known as a telescoping bolt.