Technically, any Black response other than 2...dxc4 (or another line with an early ...dxc4 that transposes into the QGA) is a Queen's Gambit Declined, but the Slav, Chigorin Defense, and Albin Counter-Gambit are generally treated separately. There are so many QGD lines after 2...e6, that many of them are distinctive enough to warrant separate treatment. The Orthodox Defense and the Tarrasch Defense are two important examples.