Simplex is a communications mode in which only one signal is transmitted, and it always goes in the same direction. The transmitter and the receiver operate on the same frequency. When two stations exist and they alternately (not simultaneously) send signals to each other on the same frequency, the mode is technically known as half duplex. However, most amateur radio operators refer to half duplex as simplex.
At high and microwave wireless frequencies, the simplex or half duplex modes will not provide enough communications range in some situations. To increase the effective range, wireless repeaters are used. In a typical repeater, the incoming signal has a different frequency than the outgoing signal, so that the transmitted signal does not overwhelm the repeater's receiver. Repeaters, strategically located in high places with large line-of-sight coverage zones, can greatly increase the range of a wireless communications system.
In order for a station to send and receive data simultaneously, two different frequencies must be used, one for transmitting and the other for receiving, a mode called full duplex. A more sophisticated mode called multiplexing allows for two or more signals or streams of information to be sent simultaneously in the same direction as an "interwoven" complex signal. The component signals are extracted and decoded individually at the receiving end.
The term simplex is sometimes used in reference to any program that resembles Simplex Database, a CGI (common gateway interface) script that provides a simple and portable database for Webmasters along with tools for creating custom Web apps.