- (Arch.) any wall surface under an arch or vault, or a painting or other decoration filling same;
- (Arch.) an opening to admit light in a dome;
- (Mil.) a fortification with two projecting faces forming a wedge plus two flanks with an open gorge;
- the hole in a guillotine in which the victim's neck is placed;
- (Farriery) a half horseshoe, lacking the ends;
- (Geol.) a broad, low-lying, typically crescent-shaped mound of sandy or loamy soil formed by the wind, especially along the windward side of a lake basin.
Plus other even more obscure meanings. In short, all sorts of things that look like a half-full or crescent moon tend to get named after the moon -- in this case, borrowing from French lunette, diminutive of lune (from Latin luna).
Above the small altar was a lunette depicting the Christ child with a lamb and a peaceable lion.