Source : Webster's Revised Unabridged Dictionary (1913)

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