Source : Webster's Revised Unabridged Dictionary (1913)

Underwood \Un"der*wood`\, n.
   Small trees and bushes that grow among large trees; coppice;
   underbrush; -- formerly used in the plural.

         Shrubs and underwoods look well enough while they grow
         within the shade of oaks and cedars.     --Addison.

Source : WordNet®

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