noun

• (countable, uncountable) A pattern along the top of a parapet (fortified wall), most often in the form of multiple, regular, rectangular spaces in the top of the wall, through which arrows or other weaponry may be shot, especially as used in medieval European architecture.
  "This villa was long and low and white, and severe after its manner : for upon and about it were none of those playful ebullitions of taste, such as conical towers, domed roofs, embattlements, statues, coloured tiles and crenellations, such as are dear to architects of villas all the world over."
• (uncountable) The act of crenellating; adding a top row that looks like the top of a medieval castle.
• (countable) Any of a series of notches with fancied resemblance to such battlements, as for example around the bezel of a flashlight.