In ShareGate Desktop, an orphaned user is a user account that is still referenced by SharePoint (the user exists in the site's user information list) even though the user can't access SharePoint anymore. This could be because the user account has been disabled or deleted from the Active Directory (or Azure Active Directory if you are using Microsoft 365).
In the case of Microsoft 365, an external user can also be an orphaned user because external users have their own account in Azure Active Directory.
Jump to: Clean Orphaned Users