We’re talking about a communications protocol, which is a common ‘language’ which allows connected devices to ‘talk’ to and understand each other. A building management system will generally use a common protocol or language between devices such as a HVAC system, lighting automation controls and access control.
BACnet (Building Automation and Control NETworks) is a network communications protocol commonly used in building automation and building management systems. It can work over both Ethernet (BACnet/IP) and serial (BACnet MS/TP) interfaces but is most often used via Ethernet. It was implemented in the mid 1990’s and is currently overseen by an organisation called BACnet International. BACnet has become one of the most common building automation protocols, along with its rival Lonworks. (link to Lonworks article)
Because BACnet is designed for building automation rather than industrial control systems, very few industrial PLC’s can converse natively in BACnet protocol. It’s usually necessary to use a converter from a more common industrial protocol such as Ethernet/IP or Modbus TCP to BACnet.