simplex
Of a circuit or device: involving signals which travel in one direction at a time; unidirectional.
Simplex: one direction at a time, full stop
A simplex circuit or device transmits signals in one direction only. When operator A sends to operator B, operator B cannot transmit back until A stops. The channel belongs entirely to whoever is transmitting at that moment. This is fundamentally different from full duplex systems, where two-way simultaneous communication happens on the same channel, and from half duplex, where both parties can transmit but not at the same time.
The clearest example is a traditional broadcast radio station. The transmitter sends continuously to thousands of receivers. Those receivers have no ability to send anything back on the same frequency. A push-to-talk walkie-talkie is half duplex: you press the button to transmit, release to listen, but you cannot do both simultaneously. Simplex is more restrictive still: one endpoint is permanently the transmitter, the other the receiver.
In control systems and industrial telemetry, simplex arrangements appear where real-time feedback is not critical. A remote sensor sending alarm signals to a central station, with no requirement for the station to query the sensor or confirm receipt, uses a simplex link. Older fire alarm systems and some SCADA networks operated this way. The trade-off is reliability: if the sender does not know whether the message arrived, you must design the system to tolerate silent failures or use redundant transmission paths.
Telegraph systems established the simplex vocabulary: signals traveled one way along the wire. When telephones arrived and required two-way talk, the industry moved to duplex and quadruplex circuits, multiplexing four independent telegraph channels on a single cable. The word simplex stuck as the term for one-directional operation, even as the underlying technology shifted from mechanical relays to vacuum tubes to semiconductors.
Modern fiber optic links can be configured simplex, half duplex, or full duplex depending on equipment at each end. A simplex fiber requires only one strand and one transmitter at one end, one receiver at the other. This saves cost and component count where unidirectional signaling is acceptable. Redundancy, however, typically means running simplex pairs, one in each direction, which negates some cost advantage.