The Collaborative International Dictionary of English v.0.48:

Thorn \Thorn\, n. [AS. [thorn]orn; akin to OS. & OFries. thorn,
   D. doorn, G. dorn, Dan. torn, Sw. t["o]rne, Icel. [thorn]orn,
   Goth. [thorn]a['u]rnus; cf. Pol. tarn, Russ. tern' the
   blackthorn, ternie thorns, Skr. t[.r][.n]a grass, blade of
   grass. [root]53.]
   1. A hard and sharp-pointed projection from a woody stem;
      usually, a branch so transformed; a spine.
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   2. (Bot.) Any shrub or small tree which bears thorns;
      especially, any species of the genus Crataegus, as the
      hawthorn, whitethorn, cockspur thorn.
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   3. Fig.: That which pricks or annoys as a thorn; anything
      troublesome; trouble; care.
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            There was given to me a thorn in the flesh, the
            messenger of Satan to buffet me.      --2 Cor. xii.
                                                  7.
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            The guilt of empire, all its thorns and cares,
            Be only mine.                         --Southern.
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   4. The name of the Anglo-Saxon letter ?, capital form ?. It
      was used to represent both of the sounds of English th, as
      in thin, then. So called because it was the initial letter
      of thorn, a spine.
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   Thorn apple (Bot.), Jamestown weed.

   Thorn broom (Bot.), a shrub that produces thorns.

   Thorn hedge, a hedge of thorn-bearing trees or bushes.

   Thorn devil. (Zool.) See Moloch, 2.

   Thorn hopper (Zool.), a tree hopper (Thelia crataegi)
      which lives on the thorn bush, apple tree, and allied
      trees.
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WordNet (r) 3.0 (2006):

thorn apple
    n 1: any of several plants of the genus Datura