Trademarked name for circular 3-pin connectors developed by Cannon (now owned by ITT). “XLR” was originally nothing more than Cannon’s part designation for the connector, which meant “X Series,” “Latch,” and “Rubber.” In fact, you’ll also sometimes see these connectors referred to as “Cannon” connectors. XLR has since evolved into a generic industry term, and many manufacturers now make this style connector. In audio work, XLR connectors are normally used for transmitting balanced mic and line level signals.
Pin 1 of an XLR connector is always ground/shield. The connectors are designed so that pin 1 makes its connection first when inserted in a jack; this ensures that the ground connection is made first, helping prevent pops and thumps in the audio chain.
Either pin 2 or pin 3 may be hot (determined by the gear the connector is plugged into), with the remaining pin being cold. To maintain correct polarity in a signal path, it is important to be aware of which pin is hot or cold on all connections, and wire your cables accordingly.