Definity

Definity is a line of digital enterprise telephone systems originally developed by AT&T as a scalable, modular private branch exchange (PBX). The platform was designed to consolidate voice switching, voicemail, and related communications functions on a single processor and was marketed through successive corporate reorganizations, passing from AT&T to Lucent Technologies and later to Avaya. In practice, Definity systems provided traditional circuit-switched trunk interfaces and internal extensions, with features such as automatic call distribution, call routing, voicemail integration, hunting, conference, and centralized management. Over time, Definity platforms evolved to support IP connectivity and integration with computer-telephone integration (CTI) and other enterprise applications, aligning with industry shifts toward converged networks.

Notable characteristics include a modular hardware architecture, multi-site operation, and the ability to run several software releases on supported server blades or cabinets. The platform was commonly deployed in mid to large-size business environments, including corporate, government, and educational settings, where reliability and feature-rich call control were valued.

In the market, Definity became part of Avaya’s enterprise portfolio, with subsequent generations transitioning toward IP-based communications platforms such as Avaya Communication Manager and related unified communications solutions. Although newer systems have emerged, Definity remains referenced in historical documentation and among long-time installations for its role in the evolution of modern IP telephony and CTI-enabled PBXs.