The Collaborative International Dictionary of English v.0.48:

catbrier \catbrier\ n.
   a very prickly woody vine of eastern U.S. (Smilax
   rotundifolia) growing in tangled masses, having tough round
   stems with shiny leathery leaves and small greenish flowers
   followed by clusters of inedible shiny black berries.

   Syn: bullbrier, greenbrier, horsebrier, brier, briar, Smilax
        rotundifolia.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

catbrier
    n 1: a very prickly woody vine of the eastern United States
         growing in tangled masses having tough round stems with
         shiny leathery leaves and small greenish flowers followed
         by clusters of inedible shiny black berries [syn:
         bullbrier, greenbrier, catbrier, horse brier,
         horse-brier, brier, briar, Smilax rotundifolia]