duplex 
(1) The ability of a printer to print on both sides of a sheet of paper, either automatically or with some manual intervention.

(2) The ability to send and receive data simultaneously. Duplex is synonymous with full duplex. For instance, when a modem operates in full-duplex mode, it can transmit data in two directions simultaneously; in half-duplex mode, in can transmit data in two directions but not simultaneously.