The term "BNC" is used to describe a "locking" coaxial impedance-matched connector used for very high frequency analog signals as well as some digital audio signals. These include Word Clock, the unbalanced professional format of the AES3 interface, MADI, and in some cases S-PDIF signals.
In order to maintain signal quality, when very high frequencies are transmitted from one device to another; the impedance of the connectors must match the characteristic impedance of the cable as well as the output and input impedance of the transmitting and receiving circuitry. This is due to the tendency for some of the signal's energy to "reflect" at an impedance miss-match in a manner analogous to the way light reflects off a partially mirrored surface (or even a clean piece of glass). The reflected energy then combines with the signal energy at the receiver and distorts the signal waveform.
BNC connectors are impedance matched to 50 or 75 Ohm cable to minimize this source of degradation. They also offer the advantage of "locking" in place to eliminate the possibility of accidental disconnection. The term "BNC" stands for "Bayonet Neill Concelman" connector but is sometimes referred to as a "Bayonet Nut Connector" as the locking mechanism is similar to that of a bayonet mount on the barrel of a gun. The cable connector has a barrel-shaped "nut" with two ramped "slots" which fit over two small "bayonet lugs" on the mating connector, allowing it to be locked in place with a quarter-turn.