Balsa is a tropical American tree, and — more often in everyday talk — the wood from that tree. The wood is famous for being extremely light: a thick piece can feel almost like foam in your hands. People who build model planes, simple rafts, or school craft projects love it because it is easy to cut and does not weigh much. It is a material word, not a poetic one. Though actually, it has nothing to do with balsam or balm, even though the spellings sit next to each other in the dictionary. English borrowed balsa from Spanish; Spanish probably took it from an old language of Spain, perhaps Basque or Iberian.