The Collaborative International Dictionary of English v.0.48:

Whitethorn \White"thorn`\, n. (Bot.)
   The hawthorn.
   [1913 Webster]

WordNet (r) 3.0 (2006):

whitethorn
    n 1: thorny Eurasian shrub of small tree having dense clusters
         of white to scarlet flowers followed by deep red berries;
         established as an escape in eastern North America [syn:
         whitethorn, English hawthorn, may, Crataegus
         laevigata, Crataegus oxycantha]