The word domain is used to refer to the root URL of a website.
Here is an example domain: semtuts.com.
What is your domain? My domain is semtuts.com.
I’m sure you get the gist, however, the domain of this URL: https://www.google.com/maps/about/ is still google.com. That’s because a domain name refers to the IP address that is being used for that particular website, and in order for a web browser to obtain access a specific URL, it will need the IP address associated with that site, therefore, it uses the domain name to do so. As the domain name has all of the information needed in order to obtain access to the web server where all of the files of the site are hosted, and those are used to display and render the site.
Some SEO’s even believe that having your primary keyword in your domain name helps with SEO, recent studies have found that exact match domains, otherwise known as EMD’s (domains that match your primary search query, exactly) were able to rank highly in the search results and surpassed non-emd’s with fewer links.
Here is an example of an exact match domain: seoservices.com