An embrasure (or crenel or crenelle; sometimes called gunhole in the domain of gunpowder-era architecture) is the opening in a battlement between two raised solid portions (merlons). Alternatively, an embrasure can be a space hollowed out throughout the thickness of a wall by the establishment of a bay. This term designates the internal part of this space, relative to the closing device, door or window. In fortification this refers to the outward splay of a window or of an arrowslit on the inside.

In ancient military engineering, embrasures were constructed in towers and walls, in particular between the merlons and the battle. A loophole, arrow loop or arrowslit passes through a solid wall, and thus forms an embrasure of shooting, allowing archer or gunner weapons to be fired out from the fortification while the firer remains under cover.

This type of opening was flared inward - that is: the opening was very narrow on the outside, but wide on the inside, so that archers had free space of movement and aiming, while exterior attackers have as much difficulty as possible to reach them. There are embrasures especially in fortified castles and bunkers. The generic term of loophole is gradually abandoned because of its imprecision, in favour of those more precise of archer, crossbowman, gunner archer. The splay of the wall on the inside provides room for defending soldiers and their equipment, allowing them to get as close to the wall-face and to the arrowslit itself as possible. Examples of deep embrasures with arrowslits are to be seen at Aigues-Mortes and Château de Coucy, both in France.

With the introduction of firearms, the term embrasure designated more specifically the opening made in a fortified structure to allow the firing of these weapons. In modern architecture, embrasures are incorporated during construction because they are intended to receive a door or a window. These are not openings made after construction.