An autonomous system is usually described as any organizational unit or scientific system which falls under the control of a single entity or is held to be in a vacuum. A city, a city's network, or an ISP would usually be described as being under the same autonomous system.
Traditionally, autonomous system is the term used by network engineers to describe a network which has a unique routing policy or is under the administrative control of a single organizational entity or individual. An AS is most often referred to in the context of the Border Gateway Protocol (BGP), where it is the primary key for describing routing announcements by a given network. The Autonomous System Number block, or ASN block, is assigned by ICANN to a given registrar and then an individual ASN is assigned to the end user. If multiple networks share the same routing policy or administrative control, separate ASNs are not usually needed. The Border Gateway Protocol requires the use of an ASN for multihoming purposes, in order to distinguish two routes which pass through or terminate into the same network
As an example, BBN/Genuity/Level(3) is the owner of AS1, since they were the first network to receive an autonomous system number from the assigning entity, which was at that time in the Internet's history the National Science Foundation. Today, it is more common to receive an ASN from ARIN, APNIC, or RIPE. Other common ASNs are AS701 (UUNet), AS1239 (Sprint), AS4200 (AGIS/Aleron), AS1668 (AOL Transit Data Network), etc. A lower ASN is considered more prestigous, but in the world of BGP it is not used in any routing decision.