The Collaborative International Dictionary of English v.0.48:

Cottonwood \Cot"ton*wood`\ (-w[oo^]d`), n. (Bot.)
   An American tree of the genus Populus or poplar, having the
   seeds covered with abundant cottonlike hairs; esp., the
   Populus monilifera and Populus angustifolia of the
   Western United States.
   [1913 Webster]