Authentication, Authorization, and Accounting: framework for intelligently controlling access to computer resources, enforcing policies, auditing usage, and providing the information necessary to bill for services.

Active Directory Domain Service: foundation for distributed networks built on Windows 2000 Server, Windows Server 2003 and Microsoft Windows Server 2008 operating systems that use domain controllers.

Active Directory Lightweight Domain Service: AD LDS is a Lightweight Directory Access Protocol (LDAP) directory service designed for use with directory-enabled applications. A directory-enabled application is one that uses a directory, as opposed to a database or flat file, for its data store.

Active Directory Migration Plan: designed to help in the upgrade or migration of active directory systems.

Asymmetric Digital Subscriber Line: transmitting digital information at a high bandwidth on existing phone lines to homes and businesses.

Asterisk Gateway Interface: is an interface for adding functionality to Asterisk with many different programming languages. Perl, PHP, C, Pascal, Bourne Shell.

Autonomous System Number: A public AS has a globally unique number, an ASN, associated with it. This number is used both in the exchange of exterior routing information (between neighboring ASes) and as an identifier of the AS itself. There are two types of ASNs: Public ASNs/Private ASNs.

Active Server Page: Microsoft's first server-side script engine for dynamically generated web pages.

AppleTalk Transaction Protocol: was the original transport-level protocol for AppleTalk, built on DDP.