In a networking context, an Autonomous System (AS) is a set of routers and networks working under control of a single and well-defined administrative unit (for instance, a Provider).
In a local context, a company’s internal network can be defined as an AS, and on a global scale geographical WANs can be considered as AS managed by a provider. In this scenario, two kinds of routers exist: internal routers exchanging information about the creation of routing tables, and edge routers which communicate with other Autonomous Systems.

Each AS is globally identified by a 16 or 32 bits ID, in practice a block of public IP addresses.