Acting captain
From Memory Alpha, the free Star Trek reference.
The position of acting captain is bestowed upon an officer or another individual aboard a starship when the ship's regular CO is unable to command for an extended period of time.
While under the influence of the polywater intoxication, Wesley Crusher deemed himself "acting captain" of the USS Enterprise-D in 2364 after commandeering the ship from engineering. (TNG: "The Naked Now")
Lieutenant Commander Data became acting captain of the Enterprise in 2367 when Captain Picard was incapacitated due to a lack of REM sleep. (TNG: "Night Terrors") Three years later, Data again became acting captain when both Captain Picard and Commander Riker were held captive by Arctus Baran. (TNG: "Gambit, Part I", "Gambit, Part II")
Aboard the mimetic USS Voyager, the Silver Blood copy of Ensign Harry Kim assumed command of the starship after his senior officers had dissolved. (VOY: "Course: Oblivion")
