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Abbreviation for private branch exchange. 1. A subscriber-owned telecommunications exchange that usually includes access to the public switched network. 2. A switch that serves a selected group of users and that is subordinate to a switch at a higher level military establishment. (188) 3. A private telephone switchboard that provides on-premises dial service and may provide connections to local and trunked communications networks. (188) Note 1: A PBX operates with only a manual switchboard; a private automatic exchange (PAX) does not have a switchboard, a private automatic branch exchange (PABX) may or may not have a switchboard. Note 2: Use of the term "PBX" is far more common than "PABX," regardless of automation.