Balsa

from 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}]

from 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]