A duplexer is a device that combines two or more signals into a common channel or medium to increase its transmission efficiency.
In radar and radio communications systems, it is a device that isolates the receiver from the transmitter while permitting them to share a common antenna. Most radio repeater systems include a duplexer.
Note 1: A duplexer must be designed for operation in the frequency band used by the receiver and transmitter, and must be capable of handling the output power of the transmitter.
Note 2: A duplexer must provide adequate rejection of transmitter noise occurring at the receive frequency, and must be designed to operate at, or less than, the frequency separation between the transmitter and receiver.
Note 3: A duplexer must provide sufficient isolation to prevent receiver desensitization.
Source: from Federal Standard 1037C
duplexer in German: Duplexer
duplexer in Japanese: デュプレクサ