A hard and sharp-pointed projection from a woody stem; usually, a branch so transformed; a spine.
- Botany A modified branch in the form of a sharp, woody spine.
- Botany Any of various shrubs, trees, or woody plants bearing sharp, woody spines.
- Any of various sharp, spiny protuberances; a prickle.
- One that causes sharp pain, irritation, or discomfort: He is a thorn in my side.
- The runic letter þ originally representing either sound of the Modern English th, as in the and thin, used in Old English and Middle English manuscripts.