What is the X in X-mas?
1570s, "semi-circular horseshoe," from Middle French lunette (13c.), literally "little moon," diminutive of lune "moon," from Latin luna (see luna). Later applied to a wide range of objects and ornamentations resembling a crescent moon.
arching aperture in a wall or concave ceiling. It may be crescent-shaped or semicircular. The word is the French diminutive of lune, "moon." Lunettes may function as windows, they may form a cove for ornament or statuary, or they may be simply a section of wall framed by an arch or vault. In the last case, the area will sometimes be decorated with a mural.