The Collaborative International Dictionary of English v.0.48:

Mistletoe \Mis"tle*toe\, n. [AS. mistelt[=a]n; mistel mistletoe
   + t[=a]n twig. AS. mistel is akin of D., G., Dan. & Sw.
   mistel, OHG. mistil, Icel. mistilteinn; and AS. t[=a]n to D.
   teen, OHG. zein, Icel. teinn, Goth. tains. Cf. Missel.]
   (Bot.)
   A parasitic evergreen plant of Europe (Viscum album),
   bearing a glutinous fruit. When found upon the oak, where it
   is rare, it was an object of superstitious regard among the
   Druids. A bird lime is prepared from its fruit. [Written also
   misletoe, misseltoe, and mistleto.] --Lindley.
   --Loudon.
   [1913 Webster]

   Note: The mistletoe of the United States is Phoradendron
         serotinum (syn. Phoradendron flavescens), having
         broader leaves than the European kind. In different
         regions various similar plants are called by this name.
         The mistletoe is used as a decoration at Christmas
         time, and it is a tradition that two persons of the
         oposite sex finding each other under a mistletoe sprig
         should kiss.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

mistletoe
    n 1: American plants closely resembling Old World mistletoe
         [syn: mistletoe, false mistletoe]
    2: Old World parasitic shrub having branching greenish stems
       with leathery leaves and waxy white glutinous berries; the
       traditional mistletoe of Christmas [syn: mistletoe, Viscum
       album, Old World mistletoe]
    3: shrub of central and southeastern Europe; partially parasitic
       on beeches, chestnuts and oaks [syn: mistletoe, Loranthus
       europaeus]