In communications, a simplex communication link is one way only, and a duplex link goes in both directions. An example of a simplex link is a satellite beaming a signal to a receiver dish where the communication only goes in one direction, from the origin to a receiver. An example of a duplex link is a telephone.
Most duplex communication links have two links: one to transmit and one to receive. A half duplex link can both receive and transmit, but it cannot do both at the same time. An example of a half duplex link is a walkie-talkie. The person talking on a walkie-talkie cannot receive while talking.
The same terminology can be used in other fields. An electrical circuit with one wire set up such that the current can only flow one way is a simplex link. Simplex fiber-optic cables have only one fiber-optic relay inside them, and duplex fiber-optic cables have two fiber-optic relays inside them.
The same terms have been borrowed for use in printing and scanning, but the terms are used somewhat incorrectly. Printers may offer the option of duplex printing, namely printing on both sides of the paper, or simplex printing, on only one side of the paper. Duplex scanners scan both sides of the paper, and simplex scanners scan only one side.