The Google definitions for "domain" are varied. Below are the definitions I picked to best explain my use of the word.
A domain is a territory over which rule or control is exercised. In Internet terms, a domain is a group of computers and devices on a network that are administered as a unit with common rules and procedures. Within the Internet, domains are defined by their Internet protocol (IP) address. All devices sharing a common part of the IP address are said to be in the same domain.