Seaborne services of the United States and many other nations refer to the officer in charge of any seagoing vessel as "captain" regardless of actual rank.
Such was the case in the 1st Mississippi Infantry which was commanded by Captain Owen Hughes after the regiment had lost over half its number at the Battle of Nashville.
Because of the ambiguity created by the common use of "captain" for officers of different grades among the Air Force, Army, Coast Guard, Marine Corps, Navy, NOAA, Public Health Service, and Space Force, equivalency is conferred between officers by use of identical pay grade rather than title of rank.
Similarly, a Navy, Public Health Service, NOAA, or Coast Guard captain is equivalent in rank, responsibilities, and grade to an Army, Air Force, Marine, or Space Force colonel, all of which are of the sixth officer grade, or O-6.
Additionally, the O-6 rank of Navy, Public Health Service, NOAA, and Coast Guard captain is abbreviated as uppercase "CAPT", while the O-3 rank of captain is abbreviated as "CPT" for the Army and mixed-case "Capt" for the Air Force, Marine Corps, and Space Force.
In the United States, the rank of captain first appeared in the Continental Army during the Revolutionary War.