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]
cottonwood, n.
1 any of several poplars, native to N. America, having seeds covered in white cottony hairs.
2 any of several trees native to Australia, esp. a downy-leaved tree, Bedfordia arborescens.