WordNet (r) 3.0 (2006):

crown of thorns
    n 1: any affliction that causes great suffering; "that is his
         cross to bear"; "he bears his afflictions like a crown of
         thorns" [syn: cross, crown of thorns]
    2: somewhat climbing bushy spurge of Madagascar having long
       woody spiny stems with few leaves and flowers with scarlet
       bracts [syn: crown of thorns, Christ thorn, Christ
       plant, Euphorbia milii]
    3: a mock crown made of thorn branches that Roman soldiers
       placed on Jesus before the Crucifixion

Easton's 1897 Bible Dictionary:

Crown of thorns
   our Lord was crowned with a, in mockery by the Romans (Matt.
   27:29). The object of Pilate's guard in doing this was probably
   to insult, and not specially to inflict pain. There is nothing
   to show that the shrub thus used was, as has been supposed, the
   spina Christi, which could have been easily woven into a wreath.
   It was probably the thorny nabk, which grew abundantly round
   about Jerusalem, and whose flexible, pliant, and round branches
   could easily be platted into the form of a crown. (See THORN
   ¯T0003642, 3.)