(n): the hard fibrous lignified substance under the bark of trees
(n): forest tree of lowland Central America having a strong very light wood; used for making floats and rafts and in crafts
(n): any tree having seeds and ovules contained in the ovary
I've never heard it used that way either, c_b. Thanks for adding it to "Out to Sea." :-)
I knew this word from childhood as a kind of wood my brother used in model airplanes, and figured it was a tree. But I never heard it used to denote a type of boat:
"'There is as odd a craft as you can possibly imagine, a balsa, I think, sailing in the strangest way... the poor soul seems to be entangled in his sheet...'"
--P. O'Brian, The Wine-Dark Sea, 222