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]

WordNet (r) 3.0 (2006):

cottonwood
    n 1: any of several North American trees of the genus Populus
         having a tuft of cottony hairs on the seed
    2: American basswood of the Allegheny region [syn: white
       basswood, cottonwood, Tilia heterophylla]