The Collaborative International Dictionary of English v.0.48:

Cadet \Ca*det"\, n. [F. cadet a younger or the youngest son or
   brother, dim. fr. L. caput head; i. e., a smaller head of the
   family, after the first or eldest. See Chief, and cf.
   Cad.]
   [1913 Webster]
   1. The younger of two brothers; a younger brother or son; the
      youngest son.
      [1913 Webster]

            The cadet of an ancient and noble family. --Wood.
      [1913 Webster]

   2. (Mil.)
      (a) A gentleman who carries arms in a regiment, as a
          volunteer, with a view of acquiring military skill and
          obtaining a commission.
      (b) A young man in training for military or naval service;
          esp. a pupil in a military or naval school, as at West
          Point, Annapolis, or Woolwich.
          [1913 Webster]

   Note: All the undergraduates at Annapolis are Naval cadets.
         The distinction between Cadet midshipmen and Cadet
         engineers was abolished by Act of Congress in 1882.
         [1913 Webster]

   3. In New Zealand, a young gentleman learning sheep farming
      at a station; also, any young man attached to a sheep
      station.
      [Webster 1913 Suppl.]

   4. A young man who makes a business of ruining girls to put
      them in brothels. [Slang, U. S.]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

cadet
    n 1: a military trainee (as at a military academy) [syn:
         cadet, plebe]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "cadet":
   Naval Reservist, Royal Marine, Seabee, bluejacket, boot, freshman,
   frogman, gob, horse marine, jolly, junior, marine, midshipman,
   midshipmite, naval cadet, navy man, plebe, senior, soph, sophomore,
   swabbie, undergrad, undergraduate, upperclassman



The Free On-line Dictionary of Computing (30 December 2018):

CADET

   Computer Aided Design Experimental Translator.

   [Sammet 1969, p. 683].

   (1994-11-29)


Bouvier's Law Dictionary, Revised 6th Ed (1856):

CADET. A younger brother, one trained up for the army or navy.