n.1. Botany a. A modified branch in the form of a sharp woody structure.
b. Any of various other sharp protuberances, such as a spine.
c. Any of various shrubs, trees, or woody plants bearing such sharp structures.
2. Any of various sharp protuberances on an animal.
3. One that causes sharp pain, irritation, or discomfort: He is a thorn in my side.
4. The runic letter þ, used in Old English, Middle English, and Old Norse manuscripts to represent both the voiceless sound (th) of Modern English thin and the voiced sound (th) of Modern English this, and in modern Icelandic orthography to represent the voiceless sound (th).