Traditionally an acronym for Private Branch eXchange. Refers to an enterprise telephony system that allows multiple telephones (terminals) to share a common set of trunk lines. Sometimes also referred to as PABX.
With innovaphone, the VoIP call server is called the PBX whereas what was known as PABX is called legacy PBX now.
In case of a distributed system, the term PBX refers to a single device with the PBX part of the firmware activated, whereas the whole PBX system is referred to as System.