Registrant

From Spamwiki

Registrant: A registrant is the person or company who purchases a domain name. For example, John Doe is a registrant who registers the name example.com through Network Solutions (the registrar)) who in turn writes the domain name to the central registry database.

Thus, a registrant is the person or entity that has the license to use a domain name by the registry of the TLD that governs the domain.

This is often referred to as the "owner" of a domain name.