A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow for simultaneous communication in both directions between two connected parties or to provide a reverse path for the monitoring and remote adjustment of equipment in the field. There are two types of duplex communication systems: full-duplex (FDX) and half-duplex (HDX).

In a full-duplex system, both parties can communicate with each other simultaneously. An example of a full-duplex device is plain old telephone service; the parties at both ends of a call can speak and be heard by the other party simultaneously. The earphone reproduces the speech of the remote party as the microphone transmits the speech of the local party. There is a two-way communication channel between them, or more strictly speaking, there are two communication channels between them.

In a half-duplex or semiduplex system, both parties can communicate with each other, but not simultaneously; the communication is one direction at a time. An example of a half-duplex device is a walkie-talkie, a two-way radio that has a push-to-talk button. When the local user wants to speak to the remote person, they push this button, which turns on the transmitter and turns off the receiver, preventing them from hearing the remote person while talking. To listen to the remote person, they release the button, which turns on the receiver and turns off the transmitter. This terminology is not completely standardized, and some sources define this mode as simplex.

Systems that do not need duplex capability may instead use simplex communication, in which one device transmits and the others can only listen. Examples are broadcast radio and television, garage door openers, baby monitors, wireless microphones, and surveillance cameras. In these devices, the communication is only in one direction.

Simplex communication is a communication channel that sends information in one direction only.

The International Telecommunication Union definition is a communications channel that operates in one direction at a time, but that may be reversible; this is termed half duplex in other contexts.

For example, in TV and radio broadcasting, information flows only from the transmitter site to multiple receivers. A pair of walkie-talkie two-way radios provide a simplex circuit in the ITU sense; only one party at a time can talk, while the other listens until it can hear an opportunity to transmit. The transmission medium (the radio signal over the air) can carry information in only one direction.

The Western Union company used the term simplex when describing the half-duplex and simplex capacity of their new transatlantic telegraph cable completed between Newfoundland and the Azores in 1928. The same definition for a simplex radio channel was used by the National Fire Protection Association in 2002.