The Collaborative International Dictionary of English v.0.48:

Midshipman \Mid"ship`man\, n.; pl. Midshipmen.
   [1913 Webster]
   1.
      (a) 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.
      (b) 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.
      (c) In the United States navy, the lowest grade of
          officers in line of promotion, being students or
          graduates of the Naval Academy awaiting promotion to
          the rank of ensign.
          [1913 Webster]

   2. (Zool.) An American marine fish of the genus Porichthys,
      allied to the toadfish; also called singingfish.
      [1913 Webster]

   Cadet midshipman, formerly a title distinguishing a cadet
      line officer from a cadet engineer at the U. S. Naval
      Academy. See under Cadet.

   Cadet midshipman, formerly, a naval cadet who had served
      his time, passed his examinations, and was awaiting
      promotion; -- now called, in the United States,
      midshipman; in England, sublieutenant.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sublieutenant \Sub`lieu*ten"ant\, n. [Pref. sub + lieutenant:
   cf. F. sous-lieutenant.]
   An inferior or second lieutenant; in the British service, a
   commissioned officer of the lowest rank.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sublieutenant
    n 1: an officer ranking next below a lieutenant