Simplex is a type of communication in which data can only be transmitted in one direction. It is often used in contrast to duplex communication, in which data can flow bidirectionally (back and forth) between two devices.
Broadcasts, in which a single transmission is sent to many users, is a common type of simplex communication. For instance, radio broadcasts, television broadcasts, and Internet streaming are all examples of simplex transmissions. Since these mediums only need to transmit, rather than receive data, simplex communication is all that is necessary.
While simplex transmissions are sufficient for broadcasting, nearly all computer network protocols and computer interfaces are duplex, meaning they can both send and receive data. Some computer components, however, may include "simplex circuits," in which data can only flow in one direction.