Commandeering was the act of taking possession or control of something, particularly for militaristic purposes. Unlike a mutiny or a hijacking, however, this was usually accomplished by asserting one's rank (sometimes over the rank of a lesser officer), and was often justifiable.
In 2267, Commodore Stocker, being delivered to Federation Starbase 10 to be its Commanding officer, took command of the USS Enterprise after Captain James T. Kirk, Commander Spock, Doctor Leonard McCoy and Chief engineering officer Montgomery Scott were affected by radiation that caused rapid aging, incapacitating them from command. However, Stocker had no field experience and, in a desperate attempt to get to Starbase 10 faster, tried to take a shortcut through the Romulan Neutral Zone, only for the ship to be surrounded and attacked by Romulan Birds-of-Prey. Once he was able to be cured of his poisoning, Kirk was able to bluff the Romulans with the Corbomite maneuver, allowing them to escape. Stocker was glad to return command to Kirk with a better understanding of what a captain was. This incident was later used as precedent in the Federation Judicial Code in 2376. (TOS: "The Deadly Years"; VOY: "Tinker Tenor Doctor Spy")
Later that year, Commodore Matt Decker took control of the USS Enterprise as the superior officer on the ship, overriding Commander Spock as acting captain, using the ship in his grief-stricken suicidal attack on a doomsday machine that killed the crew of his ship, the USS Constellation. Due to his erratic actions. Kirk used his "personal authority as captain" to get his crew to remove Decker from command before he killed them and destroyed his ship. (TOS: "The Doomsday Machine")
In 2401, former Admiral Jean-Luc Picard and former Captain William T. Riker attempted to commandeer the USS Titan-A with the idea of requesting a deviation towards the Federation border and taking a shuttle to the Ryton system. However, Captain Liam Shaw saw through their ruse and rejected their request. (PIC: "The Next Generation")