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]

The Collaborative International Dictionary of English v.0.48:

Demilune \Dem"i*lune`\, n. [F. demi- lune.]
   1. (Fort.) A work constructed beyond the main ditch of a
      fortress, and in front of the curtain between two
      bastions, intended to defend the curtain; a ravelin. See
      Ravelin.
      [1913 Webster]

   2. (Physiol.) A crescentic mass of granular protoplasm
      present in the salivary glands.
      [1913 Webster]

   Note: Each crescent is made of polyhedral cells which under
         some circumstances are supposed to give rise to new
         salivary cells.
         [1913 Webster]