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 by simply remaining 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.
The term was first used in 1690, when Lord Torrington, commander of the Royal Navy forces in the English Channel, found himself facing a stronger French fleet. He proposed avoiding a sea battle, except under very favourable conditions, until he could be reinforced. By thus keeping his 'fleet in being', he could maintain an active threat which would force the enemy to remain in the area and prevent them from taking the initiative elsewhere.
A more modern example is the stand-off between the German High Seas Fleet and the British Grand Fleet during World War I. Other than the inconclusive engagement at the Battle of Jutland, Germany preferred to keep its fleet intact rather than taking the risk of losing an engagement with the larger Royal Navy.
In World War II, actions of the Italian Royal Navy (Regia Marina) in 1940 also demonstrate the idea of a 'fleet in being'. After a number of minor battles against the Royal Navy that were mostly lost or inconclusive, the Italian fleet was left in Taranto from where it could sortie very quickly against any British attempt to reach Malta. Because of this threat the British decided to attack the Italian fleet at anchor during the famed Battle of Taranto. In this action, the British sank three Italian battleships, about half of the Italian Navy's battleship strength. Realising that Taranto harbour was no longer safe, the Italians relocated the undamaged ships to ports further away. As a 'fleet in being' is not very effective at a distance, the Italians were forced into combat and suffered repeated losses over the next two years.
Even more so than other surface vessels in the German Navy (Kriegsmarine), the powerful German battleship Tirpitz served her entire career as a 'fleet in being' in her own right. Although she never fired a shot at an enemy ship, her mere presence forced the Royal Navy to allocate powerful warships in defending Arctic convoys, and caused a major convoy (PQ-17) to scatter, suffering huge losses, mainly to U-boats and aircraft.
The 'fleet in being' concept is based on the assumption that the fleet is relatively safe in port, even if near the enemy. After the battle of Taranto and the attack on Pearl Harbor, however, it became obvious that air power made a fleet concentrated in a port vulnerable, and a fleet in being was no longer a safe option.
The idea of a 'fleet in being' can be generalised to forces other than naval. A fortress under siege is essentially an 'army in being,' which ties up enemy forces without leaving the fortress or doing much fighting. During the Gulf War, Saddam Hussein used his air force with an operational doctrine analogous to 'fleet in being.'  The mere presence of the Iraqi Air Force in hardened bunkers forced the coalition attacking Iraq to act cautiously and to escort its bomber sorties until the aircraft shelters were found to be vulnerable.