On the Internet, an autonomous system (AS) is the unit of router policy, either a single network or a group of networks that is controlled by a common network administrator (or group of administrators) on behalf of a single administrative entity (such as a university, a business enterprise, or a business division). An autonomous system is also sometimes referred to as a routing domain. An autonomous system is assigned a globally unique number, sometimes called an Autonomous System Number (ASN).
Networks within an autonomous system communicate routing information to each other using an Interior Gateway Protocol (IGP). An autonomous system shares routing information with other autonomous systems using the Border Gateway Protocol (BGP). Previously, the Exterior Gateway Protocol (EGP) was used. In the future, the BGP is expected to be replaced with the OSI Inter-Domain Routing Protocol (IDRP).
The Internet's protocol guideline for autonomous systems, after offering a definition similar to the one above, provides a more technical definition as follows:
An AS is a connected group of one or more Internet Protocol prefixes run by one or more network operators which has a SINGLE and CLEARLY DEFINED routing policy.