The Collaborative International Dictionary of English v.0.48:

markweed \markweed\ n.
   A climbing plant (Toxicodendron radicans) common in eastern
   and central U. S. with ternate leaves and greenish flowers
   followed by white berries. It yields an irritating oil that
   causes a rash on contact; commonly called poison ivy.

   Syn: poison ivy, poison mercury, poison oak, Toxicodendron
        radicans, Rhus radicans.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

markweed
    n 1: climbing plant common in eastern and central United States
         with ternate leaves and greenish flowers followed by white
         berries; yields an irritating oil that causes a rash on
         contact [syn: poison ivy, markweed, poison mercury,
         poison oak, Toxicodendron radicans, Rhus radicans]