What is the definition of Subdomain Name and Domain name?

Domain Name: Your domain name is your online identity, which allows internet users from around the world to locate and browse your web site 24 hours a day, seven days a week. It can be used as your email address and your web address.The domain name looks like: www.domainname.com

Subdomain Name: 
A subdomain is a second website, with its own unique content, but there is no new domain name. Instead, you use an existing domain name and change the www to another name. The subdomain name looks like forums.domain.com, help.domain.com.