XLR - The XLR connector is an electrical connector design. XLR plugs and sockets are used mostly in professional audio and video electronics cabling applications, often for microphones.
Home audio and video electronics normally use RCA (or phono) connectors for line level signals generated by a preamplifier - or sometimes "jack plugs". Phone plugs are also used for microphones in home and computer applications.
The names comes from its original manufacturer, James H. Cannon, founder of Cannon Electric in Los Angeles, California (now part of ITT Corporation), the connector is colloquially known as a cannon plug or cannon connector. Originally the "Cannon X" series, subsequent versions added a Latch ("Cannon XL") and then a Rubber compound surrounding the contacts, which led to the abbreviation XLR. Many companies now make XLRs. The initials "XLR" have nothing to do with the pinout of the connector. XLR connectors can have other numbers of pins besides three.