What’s in a Domain

There are 2 key Domain types you need to be familiar with in Networking

Collision Domain

Broadcast Domain

the part of a network where packet collisions can occur

Routers and Switches separate collision Domains, Hubs do not

All Devices that can reach each other with a Broadcast at Layer 2

Routers separate Broadcast Domains, Switches and Hubs do not

You could define a Switch as being a Device which creates separate Collision Domains