sharp. hard. and woody part of certain plants that is generally a modified leaf.
Merriam-Webster Online Dictionary
thorn (noun)1.
a woody plant bearing sharp impeding processes (as prickles or spines) , especially - hawthorn
2.
a) a sharp rigid process on a plant , specifically a short, indurated, sharp-pointed, and leafless modified branch
b) any of various sharp spinose structures on an animal
3.
the runic letter þ used in Old English and Middle English to represent either of the fricatives \th\ or \t͟h\ and in Icelandic to represent \th\
4.
something that causes distress or irritation - often used in the phrase thorn in one's side
thorn (Wikipedia)