Wiktionary
alt. (context military navy countable English) A naval fleet which exerts an influence while remaining in port. n. (context military navy countable English) A naval fleet which exerts an influence while remaining in port.
Wikipedia
In naval warfare, a "fleet in being" is a naval force that extends a controlling influence without ever leaving port. Were the fleet to leave port and face the enemy, it might lose in battle and no longer influence the enemy's actions, but while it remains safely in port, the enemy is forced to continually deploy forces to guard against it. A "fleet in being" can be part of a sea denial doctrine, but not one of sea control.
Usage examples of "fleet in being".
Warfare uses a concept of value which covers this situation, the fleet in being.
As they stand scattered and wounded they are not a fleet, but they are a fleet in being.
That was what they were good for now: the land equivalent of a fleet in being.