The Collaborative International Dictionary of English v.0.48:

firework \fire"work`\ (f[imac]r"w[^u]rk`), n.
   1. A device for producing a striking display of light, or a
      figure or figures in plain or colored fire, by the
      combustion of materials that burn in some peculiar manner,
      as gunpowder, sulphur, metallic filings, and various
      salts; also called a pyrotechnic device. The most common
      feature of fireworks is a paper or pasteboard tube filled
      with the combustible material. A number of these tubes or
      cases are often combined so as to make, when kindled, a
      great variety of figures in fire, often variously colored.
      The skyrocket is a common form of firework. The art of
      designing fireworks for purposes of entertainment is
      called pyrotechnics. The name firework is also given to
      various combustible preparations used in war.

   2. pl. A pyrotechnic exhibition; an entertainment consisting
      of the discharge of fireworks[1]. [Obs. in the sing.]
      [1913 Webster +PJC]

            Night before last, the Duke of Richmond gave a
            firework.                             --Walpole.
      [1913 Webster]

WordNet (r) 3.0 (2006):

firework
    n 1: (usually plural) a device with an explosive that burns at a
         low rate and with colored flames; can be used to illuminate
         areas or send signals etc. [syn: firework, pyrotechnic]