The Collaborative International Dictionary of English v.0.48:

Poison \Poi"son\, n. [F. poison, in Old French also, a potion,
   fr. L. potio a drink, draught, potion, a poisonous draught,
   fr. potare to drink. See Potable, and cf. Potion.]
   1. Any agent which, when introduced into the animal organism,
      is capable of producing a morbid, noxious, or deadly
      effect upon it; as, morphine is a deadly poison; the
      poison of pestilential diseases.
      [1913 Webster]

   2. That which taints or destroys moral purity or health; as,
      the poison of evil example; the poison of sin.
      [1913 Webster]

   Poison ash. (Bot.)
      (a) A tree of the genus Amyris (Amyris balsamifera)
          found in the West Indies, from the trunk of which a
          black liquor distills, supposed to have poisonous
          qualities.
      (b) The poison sumac (Rhus venenata). [U. S.]

   Poison dogwood (Bot.), poison sumac.

   Poison fang (Zool.), one of the superior maxillary teeth of
      some species of serpents, which, besides having the cavity
      for the pulp, is either perforated or grooved by a
      longitudinal canal, at the lower end of which the duct of
      the poison gland terminates. See Illust. under Fang.

   Poison gland (Biol.), a gland, in animals or plants, which
      secretes an acrid or venomous matter, that is conveyed
      along an organ capable of inflicting a wound.

   Poison hemlock (Bot.), a poisonous umbelliferous plant
      (Conium maculatum). See Hemlock.

   Poison ivy (Bot.), a poisonous climbing plant (formerly
      Rhus Toxicodendron, or Rhus radicans, now classified
      as Toxicodendron radicans) of North America. It is
      common as a climbing vine, especially found on tree
      trunks, or walls, or as a low, spreading vine or as a
      shrub. As a low vine it grows well in lightly shaded
      areas, recognizable by growing in clusters of three
      leaves. Its leaves are trifoliate, rhombic-ovate, and
      variously notched. Its form varies slightly from location
      to location, leading to some speculation that it may
      consist of more than one species. Many people are poisoned
      by it, though some appear resistant to its effects.
      Touching the leaves may leave a residue of an oil on the
      skin, and if not washed off quickly, sensitive areas of
      skin become reddened and develop multiple small blisters,
      lasting for several days to several weeks, and causing a
      persistent itch. The toxic reaction is due to an oil,
      present in all parts of the plant except the pollen,
      called urushiol, the active component of which is the
      compound pentadecylacatechol (according to [a

      href="http:]/www.jaxmed.com/articles/Diseases/poison_ivy_dermatitis.htm">Charles
      H. Booras). See Poison sumac. It is related to poison
      oak, and is also called mercury.

   Poison nut. (Bot.)
      (a) Nux vomica.
      (b) The tree which yields this seed (Strychnos
          Nuxvomica). It is found on the Malabar and Coromandel
          coasts.

   Poison oak (Bot.), a dermatitis-producing plant often
      lumped together with the poison ivy (Toxicodendron
      radicans) in common terminology, but more properly
      distinguished as the more shrubby Toxicodendron
      quercifolium (syn. Toxicodendron diversilobum), common
      in California and Oregon. Opinion varies as to whether the
      poison oak and poison ivy are only variants of a single
      species. See poison ivy, above.

   Poison sac. (Zool.) Same as Poison gland, above. See
      Illust. under Fang.

   Poison sumac (Bot.), a poisonous shrub formerly considered
      to be of the genus Rhus (Rhus venenata), but now
      classified as Toxicodendron vernix; -- also called
      poison ash, poison dogwood, and poison elder. It has
      pinnate leaves on graceful and slender common petioles,
      and usually grows in swampy places. Both this plant and
      the poison ivy (Toxicodendron radicans, formerly Rhus
      Toxicodendron) have clusters of smooth greenish white
      berries, while the red-fruited species of this genus are
      harmless. The tree (Rhus vernicifera) which yields the
      celebrated Japan lacquer is almost identical with the
      poison sumac, and is also very poisonous. The juice of the
      poison sumac also forms a lacquer similar to that of
      Japan.
      [1913 Webster +PJC]

   Syn: Venom; virus; bane; pest; malignity.

   Usage: Poison, Venom. Poison usually denotes something
          received into the system by the mouth, breath, etc.
          Venom is something discharged from animals and
          received by means of a wound, as by the bite or sting
          of serpents, scorpions, etc. Hence, venom specifically
          implies some malignity of nature or purpose.
          [1913 Webster]

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):

Toxicodendron radicans
    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]