What is the meaning of Crenellation?

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.

The act of crenellating; adding a top row that looks like the top of a medieval castle.

Any of a series of notches with fancied resemblance to such battlements, as for example around the bezel of a flashlight.