An apex domain is the domain name that:
For example: example.com is an apex domain.
Apex domains are also known as:
The Root domain is not a apex domain
This is not the same concept than the LDAP distinguished name (DN)
The main usage is to qualify relative name and makes them unique on an internet scope
The apex domain name can be found in:
See Domain Registration and Ownership
An apex domain is configured with one of the following DNS record:
If you're configuring the apex domain, you should also configure up the www subdomain.