Duplex

(1) A link or circuit that can carry a signal in two directions for transmitting and receiving data. (2) An optical fiber cable or cord carrying two fibers.

(2) Duplex Leads – Also known as shotgun or dual

A double loom, two lengths of cable run together to simultaneously send (TX) and receive (RX) data.
If you have a communications line that lets you send and receive data (or talk and listen) at the same time, it's duplex. If you use a cheap speakerphone, you'll notice that as soon as it picks up any noise at your end, it cuts off whatever was being said by the person on the other end. That speakerphone is not duplex.