The Collaborative International Dictionary of English v.0.48:

Machicolation \Mach`i*co*la"tion\, n. [Cf. LL. machicolamentum,
   machacolladura, F. m[^a]chicolis, m[^a]checoulis; perh. fr.
   F. m[`e]che match, combustible matter + OF. coulis, couleis,
   flowing, fr. OF. & F. couler to flow. Cf. Match for making
   fire, and Cullis.]
   [1913 Webster]
   1. (Mil. Arch.) An opening between the corbels which support
      a projecting parapet, or in the floor of a gallery or the
      roof of a portal, for shooting or dropping missiles upon
      assailants attacking the base of the walls. Also, the
      construction of such defenses, in general, when of this
      character. See Illusts. of Battlement and Castle.
      [1913 Webster]

   2. The act of discharging missiles or pouring burning or
      melted substances upon assailants through such apertures.
      [1913 Webster]

WordNet (r) 3.0 (2006):

machicolation
    n 1: a projecting parapet supported by corbels on a medieval
         castle; has openings through which stones or boiling water
         could be dropped on an enemy