The Bayonet Neill-Concelman (sometimes wrongly called the British Naval Connector) is a type of RF connector used for terminating coax cable. Some authorities expand the abbreviation as Baby Neill-Concelman - it is a lot smaller than N and C connectors.
The BNC connector is one of a larger class of "bayonet connectors", named after the resemblance to the standard twist-on attachment for a bayonet. Named after Paul Neill of Bell Labs (inventor of the N connector) and Amphenol engineer Carl Concelman (inventor of the C connector), the BNC was originally designed as a miniature form of the Type C connector.
A threaded version of the BNC connector, known as the TNC connector (for Threaded Neill-Concelman) is also available. It has superior performance to the BNC connector at microwave frequencies.