The Collaborative International Dictionary of English v.0.48:

Half-moon \Half"-moon`\ (-m[=oo]n`), n.
   1. The moon at the quarters, when half its disk appears
      illuminated.
      [1913 Webster]

   2. The shape of a half-moon; a crescent.
      [1913 Webster]

            See how in warlike muster they appear,
            In rhombs, and wedges, and half-moons, and wings.
                                                  --Milton.
      [1913 Webster]

   3. (Fort.) An outwork composed of two faces, forming a
      salient angle whose gorge resembles a half-moon; -- now
      called a ravelin.
      [1913 Webster]

   4. (Zool.) A marine, sparoid, food fish of California
      (C[ae]siosoma Californiense). The body is ovate,
      blackish above, blue or gray below. Called also
      medialuna.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ravelin \Rave"lin\ (r[a^]v"l[i^]n; 277), n. [F.; cf. Sp.
   rebellin, It. revellino, rivellino; perhaps fr. L. re- again
   + vallum wall.] (Fort.)
   A detached work with two embankments which make a salient
   angle. It is raised before the curtain on the counterscarp of
   the place. Formerly called demilune and half-moon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

half-moon
    n 1: the crescent-shaped area at the base of the human
         fingernail [syn: half-moon, lunula, lunule]
    2: the time at which the Moon is at first or last quarter when
       half its face is illuminated