Source : Webster's Revised Unabridged Dictionary (1913)

Defoliation \De*fo`li*a"tion\, n. [LL. defoliare, defoliatum, to
   shed leaves; L. de- + folium leaf: cf. F. d['e]foliation.]
   The separation of ripened leaves from a branch or stem; the
   falling or shedding of the leaves.

Source : WordNet®

defoliation
     n 1: the loss of foliage
     2: causing the leaves of trees and other plants to fall off (as
        by the use of chemicals)