machicolation

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A projecting gallery at the top of a castle wall, supported by a row of corbeled arches and having openings in the floor through which stones and boiling liquids could be dropped on attackers.
2. n. One of these openings.
3. n. A row of small corbeled arches used as an ornamental architectural feature.

Century Dictionary and Cyclopedia

1. n. In medieval architecture, an opening in the vault of a portal or passage, or in the floor of a projecting gallery, made for the purpose of hurling missiles, or pouring down molten lead, hot pitch, etc., upon an enemy essaying to enter or mine. In the gallery type machicolations are formed by setting out the parapet or breastwork, B, supported on corbels; beyond the face of the wall, G, spaces between the corbels are left open, and constitute the machicolations. (See cut on following page.) Machicolations of permanent construction in stone were not introduced until toward the end of the twelfth century; but in the hoarding of wood with which walls and towers were crowned in time of need from the earliest period of the middle ages, their use was constant.
2. n. The act of hurling missiles or of pouring burning liquids upon an enemy through apertures such as those described above.
3. n. By extension, a machicolated parapet or gallery, or a projection supported on corbels, in imitation of medieval machicolated construction, without openings.

Wiktionary

1. n. An opening between the corbels which support a projecting parapet, or in the floor of a gallery or the roof of a portal, shooting or dropping missiles upon assailants attacking the base of the walls. Also, the construction of such defenses, in general, when of this character.
2. n. The act of discharging missiles or pouring burning or melted substances upon assailants through such apertures.

GNU Webster's 1913

1. n. (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.
2. n. The act of discharging missiles or pouring burning or melted substances upon assailants through such apertures.

WordNet 3.0

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