1 barb, spine, spike, prickle, bristle, brier, bur or burr, point, bramble, cocklebur:
My sleeve caught on the thorn and tore.
2 Often, thorn in (one's) side. bother, irritation, annoyance, nuisance, vexation, torment, torture, scourge, plague, affliction, irritant, bane, Colloq pain in the neck, Taboo Slang pain in the Brit arse or US ass:
His nagging brother has been a thorn in his side for years.