A dormant account is a login account which has not had any login activity for an extended period of time, defined by being at least some N number of days.
Dormant accounts are generally assumed to be unneeded -- no use implies no need. Dormant accounts represent a security risk since any unusual use of the account will not be detected by the account's (inactive) owner.
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.