Missing in action, or MIA, is a status assigned to a member of Starfleet whose current whereabouts and condition is unknown to Starfleet. Such members may have been injured, captured by a hostile force, or killed.
In one instance the missing in action status was assigned to Captain Willard Decker and Lieutenant Ilia after they had joined with the V'Ger entity to create a new life form. Admiral James T. Kirk felt that neither officer was dead, and that to list them as MIA was more appropriate rather than listing them as deceased. (TOS movie: Star Trek: The Motion Picture)