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]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "cornet":
   English horn, alpenhorn, alphorn, althorn, alto horn, ballad horn,
   baritone, bass horn, bassoon, block flute, bombard, bourdon,
   brass choir, brass wind, brass-wind instrument, brasses, bugle,
   bugle horn, cello, claribel, clarinet, clarion, complex cone,
   concert flute, cone, conelet, conoid, cop, cornet-a-pistons,
   corno di caccia, cornopean, cromorna, cymbel, diapason,
   double-bell euphonium, dulciana, euphonium, flute stop,
   foundation stop, fourniture, funnel, gamba, gedeckt, gemshorn,
   harmonic flute, helicon, horn, hunting horn, hybrid stop,
   ice-cream cone, key trumpet, koppel flute, larigot, lituus, lur,
   mellophone, melodia, mixture, mutation stop, nazard, oboe, octave,
   ophicleide, orchestral horn, organ stop, piccolo, pine cone,
   plein jeu, pocket trumpet, posaune, post horn, principal, quint,
   quintaten, rank, ranket, reed stop, register, rohr flute, sackbut,
   saxhorn, saxtuba, serpent, sesquialtera, shawm, slide trombone,
   sliphorn, sousaphone, spitz flute, stop, stopped diapason,
   stopped flute, string diapason, string stop, tenor tuba, tierce,
   tremolo, tromba, trombone, trumpet, tuba, twelfth, unda maris,
   valve trombone, valve trumpet, vibrato, viola, voix celeste,
   vox angelica, vox humana



Easton's 1897 Bible Dictionary:

Cornet
   Heb. shophar, "brightness," with reference to the clearness of
   its sound (1 Chr. 15:28; 2 Chr. 15:14; Ps. 98:6; Hos. 5:8). It
   is usually rendered in the Authorized Version "trumpet." It
   denotes the long and straight horn, about eighteen inches long.
   The words of Joel, "Blow the trumpet," literally, "Sound the
   cornet," refer to the festival which was the preparation for the
   day of Atonement. In Dan. 3:5, 7, 10, 15, the word (keren) so
   rendered is a curved horn. The word "cornet" in 2 Sam. 6:5 (Heb.
   mena'an'im, occurring only here) was some kind of instrument
   played by being shaken like the Egyptian sistrum, consisting of
   rings or bells hung loosely on iron rods.