Are you trying to simplify its definition as much as possible, we could say that a domain name registration is a name that serves to uniquely identify a set of computers connected to the internet. Other common ways to call a domain are domain name or internet domain.
What are Domain Names for?
One of the reasons why domain names were created was to make it easier for people to remember the name of a set of computers connected to the Internet. This is because, on the Internet, a computer is uniquely identified by an IP address.
An IP address is just a number. Thus, in the case of the IPv4 protocol (IP version 4), an IP address is a 32-bit number that is usually represented in decimal notation by 4 numbers separated by periods, where each of these numbers is within the range between 0 and 255 (both inclusive). For example, 255.255.255.255, 184.108.40.206 or 127.0.0.1 are IPv4 addresses.