The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   2. the act or process of causing plants to lose their leaves,
      especially by application of a chemical agent.

   Note: The deliberate defoliation of plants has been used in
         war (as in Vietnam) to deprive an enemy of cover and
         allow attack from the air; also, to destroy
         narcotic-producing plants as a tactic against illegal
         drug production. The chemical defoliating agents are
         often sprayed over large areas from airplanes.
         [PJC]

WordNet (r) 3.0 (2006):

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)