noun

• (architecture) An opening between corbels that support a projecting parapet, or in the floor of a gallery or the roof of a portal, of a fortified building from which missiles can be shot or heated items dropped upon assailants attacking the base of the walls.
  "Leibourn Caſtle, Kent. A proſpect of the front of that caſtle. Over the gate, we are told, was a machicolation or contrivance, from whence, in caſe of a ſudden attack, great ſtones, boiling water, or melted lead, might be thrown down upon the aſſailants."
• (architecture) A projecting parapet with a series of such openings.
  "The main walls were carried to the top of the fourth story, where a capacious machicolation enclosed the tower, on which there is a parapet wall of great thickness, with arches: this was to protect the persons employed over the machicolations."