<communications> (Obsolete: "half-duplex") A mode of operation of a communications program or device in which it displays the characters the user enters at the same time as it sends them to the remote system.
In communications between computers or computing processes, particularly those involving human keyboarding and/or reading, duplex came to mean the re-transmission of a keyboard character to the output display.
Early input device such as the Teletype ASR-33 teleprinter, being descended from the electric typewriter, printed all input characters as they were typed (i.e. they did local echo). Local echo was typically optional on the video terminals that replaced them, and usually disabled in favour of remote echo. A disadvantage of local echo is that it will continue, even when the communication circuit has failed, which can be misleading.