The Collaborative International Dictionary of English v.0.48:

Gunpowder \Gun"pow`der\ (g[u^]n"pou`d[~e]r), n. (Chem.)
   A black, granular, explosive substance, consisting of an
   intimate mechanical mixture of saltpeter, charcoal, and
   sulphur. It is used in gunnery and blasting.
   [1913 Webster]

   Note: Gunpowder consists of from 70 to 80 per cent of
         potassium nitraate (niter, saltpeter), with 10 to 15
         per cent of each of the other ingredients. Its
         explosive energy is due to the fact that it contains
         the necessary amount of oxygen for its own combustion,
         and liberates gases (chiefly nitrogen and carbon
         dioxide), which occupy a thousand or fifteen hundred
         times more space than the powder which generated them.
         [1913 Webster]

   Gunpowder pile driver, a pile driver, the hammer of which
      is thrown up by the explosion of gunpowder.

   Gunpowder plot (Eng. Hist.), a plot to destroy the King,
      Lords, and Commons, in revenge for the penal laws against
      Catholics. As Guy Fawkes, the agent of the conspirators,
      was about to fire the mine, which was placed under the
      House of Lords, he was seized, Nov. 5, 1605. Hence, Nov. 5
      is known in England as

   Guy Fawkes Day.

   Gunpowder tea, a species of fine green tea, each leaf of
      which is rolled into a small ball or pellet.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Guy Fawkes Day
    n 1: day for the celebration of the discovery of the Gunpowder
         Plot