an autonomous system

The Internet is not one giant network run by one company. It is tens of thousands of separate networks that have agreed to carry each other's traffic. Each of those networks is an autonomous system, or AS. Think of the Internet as a continent of independent towns: each town runs its own roads however it likes, but they all build connecting highways so anyone can drive between them. An AS is one of those towns.

More precisely, an autonomous system is a collection of routers and the IP address ranges they own, all run under a single administrative authority and a single, consistent routing policy. A big university, a phone company, a cloud provider like Amazon, or a content company like Netflix might each be one AS (large operators sometimes run several). Inside its own borders the AS chooses its own routing protocol — maybe OSPF, maybe something else — and that is its private business. What matters to the outside world is which IP prefixes the AS can reach, and how it advertises them to its neighbours.

The whole point of the word autonomous is local control. Nobody outside the AS gets to dictate how it routes internally or which neighbours it prefers; the AS decides. The Internet's inter-domain routing system, the Border Gateway Protocol, exists precisely to glue these autonomous, self-governing networks together without any of them having to surrender that control.