An orphaned user is a user account still referenced by SharePoint even though the user cannot access SharePoint anymore.
When you disable or delete a user from your Active Directory (AD), that user's permissions and references in metadata fields such as Created by and Modified by do not get removed in SharePoint or Microsoft 365, and that user becomes an orphaned user.
Note: Because external users are in the Azure Active Directory in Microsoft 365, an external user can also be an orphaned user.
To learn why you should remove orphaned user permissions and how to remove them, see Clean orphaned users.