Source : Webster's Revised Unabridged Dictionary (1913)

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 {P.
   monilifera} and {P. angustifolia} of the Western United
   States.

Source : WordNet®

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}, {Tilia heterophylla}]