em·bra·sure /ɛmˈbreɪʒər/
1. (in fortification) an opening, as a loophole or crenel, through which missiles may be discharged.
2. Architecture. a splayed enlargement of a door or window toward the inner face of a wall.
3. Dentistry. the space between adjacent teeth.

[Origin: 1695-1705; < F, equiv. to embras(er) to enlarge a window or door opening, make an embrasure (appar. the same v. as embraser to set on fire (see embrace²), though sense shift unclear) + –ure -ure]

-Related forms
em·bra·sured, adjective

Source: Dictionary.com.

Two straight and unadorned columns stood on either side of the entrance, which opened, at first sight, like a single great arch; but from the columns began two embrasures that, surmounted by other, multiple arches, led the gaze, as if into the heart of an abyss, toward the doorway itself, crowned by a great tympanum, supported on the sides by two imposts and in the center by a carved pillar, which divided the entrance into two apertures protected by oak doors reinforced in metal.

Source: The Name of the Rose, Umberto Eco.