A reserve fleet is a collection of naval vessels of all types that are fully equipped for service but are not currently needed, and thus partially or fully decommissioned. A reserve fleet is informally said to be "in mothballs" or "mothballed"; an equivalent expression in unofficial modern U.S. naval usage is "ghost fleet". In earlier times, and especially in British usage, these ships were said to be laid up in ordinary.
Such ships are 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 naval bases or shipyards to speed the reactivation process.