Midshipman

Definition:

(n.) Formerly, a kind of naval cadet, in a ship of war, whose business was to carry orders, messages, reports, etc., between the officers of the quarter-deck and those of the forecastle, and render other services as required.
(n.) In the English naval service, the second rank attained by a combatant officer after a term of service as naval cadet. Having served three and a half years in this rank, and passed an examination, he is eligible to promotion to the rank of lieutenant.
(n.) In the United States navy, the lowest grade of officers in line of promotion, being graduates of the Naval Academy awaiting promotion to the rank of ensign.
(n.) An American marine fish of the genus Porichthys, allied to the toadfish.

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midshipman vs. midshipmen