The admin domain resembles the name of your site, which is automatically generated while creating your site. This domain will always be available even after you create a custom domain or custom URL for your site. The domain needs to be renewed when/before expired to keep your site seamlessly working.
The admin domain consists of the name of your site plus the suffix '.ghoststead.net'. So if you name your site
example then your admin domain would be
The admin domain can be reached through its URL. The exact URL depends on whether or not you use a custom URL and if that URL has extra path information. For instance, this site is (of course) a Ghoststead help center site with the name
ghoststead-helpcenter with and admin domain of
ghoststead-helpcenter.ghoststead.net. The custom URL is https://www.ghoststead.com/help/ since it is integrated into the main ghoststead.com site. This makes the site discoverable using the admin domain through the URL https://ghoststead-helpcenter.ghoststead.com/help/.
NOTE: Both of these URLs refer to the same site but the non admin domain URL is the canonical one.