The development of the PBX system for businesses progressed rapidly in the 1980s, with Avaya (at the time, part of Lucent Technologies) Definity being the final model and family put out by the company in that time period. At the time, the Definity was the market leader in features, and gained significant penetration in the market, with many businesses choosing it over the Merlin Legend system put out by the same company, largely because the Definity was the first phone system to use VoIP technology at the time.

The Definity system was also capable of several hundred more seats than the Lucent Merlin system, and for larger organizations became the default standard; it also represented a significant improvement in telephone capabilities. In the 1990s, external competition forced a price war which brought the Definity system to an even wider market, as the feature set expanded.

The Definity system requires an extensive investment to install - not in the hardware, which is relatively inexpensive, but in the systems integration level. It takes significant technical expertise to roll out a Definity system from a cold start. That said, for users of the Definity and its immediate predecessors, the System 75 and System 85, the Definity system provides an excellent scalable upgrade path, covering multiplex phone systems with voice mail features, remote call forwarding and the ability to handle email and multimedia integration. The latest Definity services package can handle over 700 telephony features. It's important to note that the Definity and the Merlin systems use incompatible protocols on their back end; an environment that's using one or the other has a difficult time integrating the two, and it may be simpler to replace a Merlin solution with a Definity one in the long run if an upgrade is needed.

The current state of the art in the Definity family of products is the Avaya Communications Manager, with Avaya Media Servers and Avaya Media Gateways; these are blade mounted servers in a PBX chassis and are backwards compatible with existing Definity hardware, allowing for upgrading as needed. The current generation of Definity servers run off of x86 hardware platforms, and run on the Linux family of operating systems.

There are a number of after market vendors working with Avaya as resellers of both hardware and installation and support services. Many specialize in the Avaya Definity upgrade path, and can provide extensive resources to make the transition to Definity easier, or help you get the tailored solution that you need to meet your business's telephony needs.

Many Avaya distributors are well versed in the latest iteration of the Definity solution services set: Avaya Aura, which is Avaya's attempt to upgrade the software package running on the existing hardware services to improve ease of use, add additional user and manager-level features and make transitions to the new platform easier.

Service and installation options for the Avaya Definity Phone Systems are offered by many authorized distributors.