Source : Webster's Revised Unabridged Dictionary (1913)

Underbrush \Un"der*brush`\, n.
   Shrubs, small trees, and the like, in a wood or forest,
   growing beneath large trees; undergrowth.

Source : WordNet®

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