Crenellation is the functional or decorative pattern of notches in a parapet, originally used in defensive architecture to provide cover for defenders while they could fire arrows, lances, or firearms or launch rocks or missiles through the gaps at invaders.

Although both the notches and the shielding elements are typically squared off, they can also be pointed or take various shapes.

Also known as battlements, crenellation was originally developed in the Middle Ages for castles, fortresses, guard towers, and defensive walls, it can frequently be found as a decorative element in structures beginning in the 19th century, especially neo-gothic and tudor revival residential buildings, palaces, civic structures, and churches.