The Collaborative International Dictionary of English v.0.48:

Cornet \Cor"net\ (k?r"n?t), n. [F. cornet, m. (for senses 1 &
   2), cornette, f. & m. (for senses 3 & 4), dim. of corne horn,
   L. cornu. See Horn.]
   1. (Mus.)
      (a) An obsolete rude reed instrument (Ger. Zinken), of the
          oboe family.
      (b) A brass instrument, with cupped mouthpiece, and
          furnished with valves or pistons, now used in bands,
          and, in place of the trumpet, in orchestras. See
          Cornet-[`a]-piston.
      (c) A certain organ stop or register.
          [1913 Webster]

   2. A cap of paper twisted at the end, used by retailers to
      inclose small wares. --Cotgrave.
      [1913 Webster]

   3. (Mil.)
      (a) A troop of cavalry; -- so called from its being
          accompanied by a cornet player. [Obs.] "A body of five
          cornets of horse." --Clarendon.
      (b) The standard of such a troop. [Obs.]
      (c) The lowest grade of commissioned officer in a British
          cavalry troop, who carried the standard. The office
          was abolished in 1871.
          [1913 Webster]

   4. A headdress:
      (a) A square cap anciently worn as a mark of certain
          professions.
      (b) A part of a woman's headdress, in the 16th century.
          [1913 Webster]

   5. [Cf. Coronet.] (Far.) See Coronet, 2.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cornet
    n 1: a brass musical instrument with a brilliant tone; has a
         narrow tube and a flared bell and is played by means of
         valves [syn: cornet, horn, trumpet, trump]