balsa
Concise Oxford English Dictionary © 2008 Oxford University Press:
balsa /ˈbɒlsə/
▶noun
- 1 (also balsa wood) very lightweight timber used chiefly for making models and rafts.
- 2 the fast-growing tropical American tree from which balsa wood is obtained. [Ochroma lagopus.]
– origin C17: from Sp., ‘raft’.
'balsa' also found in these Oxford entries:

