What does domain mean?
The root address for a website or web page. The domain is the element of a DNS (domain name system) that indicates the IP address of a website or a computer. When more than one computer shares a common part of the same IP address they are said to be in the same domain.
To understand the meaning of the domain name system, you can think of a tree that has several branches. The domain would be a limb of the tree that incorporates all the branches of the tree that originate from it.