Definition - What does User Principal Name (UPN) mean?
User Principal Name (UPN) is a term for a username in "email format" for use in Windows Active Directory. Here, the user’s personal username is separated from a domain name by the traditional "@" sign.
Techopedia explains User Principal Name (UPN)
User Principal Name is used to authenticate users on the Windows OS. It can replace other aspects of usernames within a Windows profile. It can also be used to abbreviate some long domain name lists.
The UPN is also used to obtain the account of a user in the Active Directory, which can be queried using the Active Directory Service Interface. This is a type of library that provides an interface to different kinds of directories, such as the user account database used by Windows.