A dormant user is an identity where all of the accounts associated with the identity are dormant. Presumably the user is completely inactive -- since he is not signing into any of his accounts. Accounts belonging to inactive users should generally be disabled and (later) deleted.
Dormant users are related to, but not the same as, orphan users. Orphan users are users whose relationship to the organization is not documented, while dormant users are inactive.
Hitachi ID Identity Manager can be used to find orphan and dormant accounts:
The lists of dormant and orphan accounts generated in this way are tentative and should not in general be automatically disabled. For example, apparently-dormant accounts may simply be infrequently used, while apparently-orphan accounts may simply not yet have been attached to their owner's profile.
Orphan and dormant account lists can and should be manually reviewed, to remove obvious errors. The resulting, sanitized lists should be resubmitted to Identity Manager first to batch-disable, and later to batch-delete.
The time interval between disabling and deleting orphan accounts gives the owners of those accounts time to notice the problem and complain, thereby causing their accounts to be reactivated.