Acting first officer
The position of acting first officer was bestowed upon an officer or another individual aboard a starship when the ship's regular first officer was unable to fulfill his duties or until a permanent replacement had been found. 

In an alternate 2258, created by the Romulan Nero, Captain  of the  appointed Commander  acting captain and Lieutenant  as first officer when Pike left the ship to confront Nero. Subsequently, Kirk proved that Spock was unable to perform his duties and then Kirk himself was made acting captain. After everything was over, Kirk was promoted to the rank of captain and given regular command of the ship. ()

In 2259, Lieutenant commander La'an Noonien-Singh was made the acting first officer of the  after its regular first officer, Una Chin-Riley went missing on Kiley 279. ()

Lieutenant Commander Data was made the acting first officer after the disappearance of Captain Jean-Luc Picard in 2370. ()

 Tom Paris filled the role of acting first officer after Captain Kathryn Janeway was assimilated by the Borg in 2377. ()

Ensign Sylvia Tilly filled the role of acting first officer after Commander Michael Burnham was demoted from the position for insubordination in 3189. ()

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