1. The moon at the quarters, when half its disk appears illuminated.
2. The shape of a half-moon; a crescent. See how in warlike muster they appear, in rhombs, and wedges, and half-moons, and wings. (Milton)
3. An outwork composed of two faces, forming a salient angle whose gorge resembles a half-moon; now called a ravelin.
4. (Science: zoology) a marine, sparoid, food fish of California (Caesiosoma Californiense). The body is ovate, blackish above, blue or gray below.