The Collaborative International Dictionary of English v.0.48:

Baleen \Ba*leen"\ (b[.a]*l[=e]n"), n. [F. baleine whale and
   whalebone, L. balaena a whale; cf. Gr. fa`laina. ] (Zool. &
   Com.)
   Plates or blades of "whalebone," from two to twelve feet
   long, and sometimes a foot wide, which in certain whales
   (Bal[ae]noidea) are attached side by side along the upper
   jaw, and form a fringelike sieve by which the food is
   retained in the mouth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

baleen
    n 1: a horny material from the upper jaws of certain whales;
         used as the ribs of fans or as stays in corsets [syn:
         whalebone, baleen]