The Collaborative International Dictionary of English v.0.48:

Balsa \Bal"sa\, n. [Sp. or Pg. balsa.] (Naut.)
   A raft or float, used principally on the Pacific coast of
   South America.
   [1913 Webster]

WordNet (r) 3.0 (2006):

balsa
    n 1: strong lightweight wood of the balsa tree used especially
         for floats [syn: balsa, balsa wood]
    2: forest tree of lowland Central America having a strong very
       light wood; used for making floats and rafts and in crafts
       [syn: balsa, Ochroma lagopus]