an asynchronous communications protocol in which the communications channel can handle only one signal at a time - the communicating stations alternate between transmission and reception
Simultaneous send and receive of data but at different speeds, details ...
the inability to send and receive data simultaneously which, in digital audio terms, translates to the inability to record and play audio at the same time. Many older sound cards are half-duplex. Most modern sound cards are capable of recording and playing audio simultaneously. This capability is called 'full-duplex'.
Describes a circuit capable of transmitting in either direction, but only one direction at a time. See also: Full Duplex, Synchronous.
One way at a time data communication; both devices can transmit and receive data, but only one at a time. 
Data transfer that only sends or receives data to a node. Hubs commonly perform in half duplex mode.
Capability for data transmission in only one direction at a time between a sending station and a receiving station. Compare with full duplex and simplex.