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):

Phoradendron serotinum
    n 1: the traditional mistletoe of Christmas in America: grows on
         deciduous trees and can severely weaken the host plant
         [syn: American mistletoe, Phoradendron serotinum,
         Phoradendron flavescens]