The Collaborative International Dictionary of English v.0.48:

Undergrowth \Un"der*growth`\, n.
   That which grows under trees; specifically, shrubs or small
   trees growing among large trees. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undergrowth
    n 1: the brush (small trees and bushes and ferns etc.) growing
         beneath taller trees in a wood or forest [syn:
         underbrush, undergrowth, underwood]