The Definity was the last in a line of circuit based PBX hardware manufactured since the late 1980s by Avaya, then the former Enterprise Networks division of Lucent and AT&T's Information Systems unit. The original Definity servers and switches can be enabled to use some versions of Avaya's later Communication Manager (now known as the Aura) software using a Media Server to control a Definity switch (or a media gateway in recent releases). The Definity/Communication Manager has about 700 telephony features, one of the largest feature-sets compared to a Nortel Meridian, NEC's NEAX or Cisco's CallManager.

The Definity servers prior to Communication Manager use a proprietary operating system called Oryx/Pecos. The call processing, tone clocks, trunk and phone terminal connections as, well as other digital processing technologies was processed through circuit cards to ensure reliable telephone conversations. The most used setups for a Definity based system would range from small colleges, hospitals, public safety, large corporations or government, department stores, and specific locations that use call centers, due to programming and administration and user interface that can meet a need for call center applications.

The Definity switches through the respected variants of the systems for quite some time sold ether in a single carrier cabinet (SCC) or a multi carrier cabinet (MCC.) SCC had about 14 slots and a user count up to about 120, and could be daisy chained to up to 5 carriers per node to about 450 users total per node. The MCC worked as a large mainframe, premise based carrier switch. The MCC contained about 200 slots for a total about 650 users per node. (In Aura Release 5, the SCC and MCC are officially discontinued for software support, with requirement to use a rack formed media gateway like the SCC.)

The primary switching unit was the Processor Port Network (PPN) that handled the Oryx/Pecos, and other essential services for the system. For satellite locations within a campus or elsewhere, an Expanded Port Network (EPN) would be used. This would act as a dumb switch, requiring the PPN to operate most of all the features. The respected switching unit could be used on a respective MCC/SCC carriers.