AS = Autonomous System Number
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information between autonomous systems (AS) on the Internet. The protocol is often classified as a path vector protocol, but is sometimes also classed as a distance-vector routing protocol. The Border Gateway Protocol makes routing decisions based on paths, network policies or rule-sets configured by a network administrator, and is involved in making core routing decisions.
BGP may be used for routing within an AS. In this application it is referred to as Interior Border Gateway Protocol, Internal BGP or iBGP. In contrast, the Internet application of the protocol may be referred to as Exterior Border Gateway Protocol, External BGP or EBGP.