The term BNC “T” is used to describe a special BNC adapter with three connectors which is typically use to “split” or “mult” a BNC connection.
The BNC T is required to connect a BNC Terminator to the end of a cable that connects to a BNC input. By providing the necessary extra connection, the cable can be connected to both the input it is feeding and the terminator. Like all BNC connectors, the BNC T has characteristic impedance that matches the cable and terminator. For connections use in digital audio this impedance is 75 Ohms. The other common impedance for BNC connectors is 50 Ohms. For best performance, it is important that the BNC T be of 75 Ohm impedance. Please see termination more information.