What is the mean by Domain in Network?

Domain which resolve IP address to host name and host name to IP.

A simple view of a domain is a central database that allows users to connect to different network resources using the same credentials (username and passwords). Of course, all resources will need to be part of the domain and there is a lot more a domain can do. The alternative to the domain model is the workgroup model which requires you to create a username and password on every network resource you are trying to access and which can get cumbersome as servers and users increase. What vilslifestyle is talking about is something different... which is DNS or Domain Naming System.