A reserve fleet or (less formally) mothball fleet is a collection of Starfleet vessels (both warships and support vessels) that are fully equipped for service but are not currently needed and thus partially or fully decommissioned. In earlier times and especially in Starfleet usage, these ships were said to be laid up in ordinary, whilst a similar phrase in unofficial modern Starfleet naval parlance is ghost fleet.
Such ships are generally held in reserve against a time when it may be necessary to call them back into service, and are usually tied up in backwater areas near Starfleet bases or shipyards to speed the reactivation process. They may be modified—for instance, by having rust prone areas sealed off or wrapped in plastic (or, in the case of warships, removal of the masts). While held in the reserve fleet, the ships will typically have a minimal crew (or, less formally, a skeleton crew) that makes sure the ship stays in usable condition. When a ship is placed in reserve status, the various parts and weapon systems that the vessel uses are also placed in a storage facility so that if the ship is ever reactivated, the proper spare parts and ammunition can be loaded aboard the vessel—though like the ships themselves, these stored parts and equipment are prone to fall into disrepair and obsolescence