What does "duplex" mean?

Duplex: Double; twofold. Duplex escapement, a peculiar kind of watch escapement, in which the scape-wheel has two sets of teeth. See Escapement. -- Duplex lathe, one for turning off, screwing, and surfacing, by means of two cutting tools, on opposite sides of the piece operated upon. -- Duplex pumping engine, a steam pump in which two steam cylinders are placed side by side, one operating the valves of the other. -- Duplex querela Etym: [L., double complaint] (Eccl. Law), a complaint in the nature of an appeal from the ordinary to his immediate superior, as from a bishop to an archbishop. Mozley & W. -- Duplex telegraphy, a system of telegraphy for sending two messages over the same wire simultaneously. -- Duplex watch, one with a duplex escapement.