An Autonomous System (AS) is a network under single administrative control with a unified routing policy.
Examples:
- A large ISP like Comcast
- A content provider like Netflix
- A university network
- A multinational corporation
What defines an AS:
- Single administrative authority
- Consistent internal routing policy
- Unified external view
Think of an AS as one "routing entity" on the internet. Internally it might run OSPF. Externally it speaks BGP. The boundary determines who you exchange routes with.