thorn

noun

• 1a stiff, sharp-pointed, straight or curved woody projection on the stem or other part of a plant.
• a source of discomfort, annoyance, or difficulty; an irritation or an obstacle:the issue has become a thorn in renewing the peace talksSee also a thorn in someone's side below.

• 2 (also thorn bush or thorn tree) a thorny bush, shrub, or tree, especially a hawthorn.

• 3an Old English and Icelandic runic letter, Þ or þ, representing the dental fricatives /T͟H/ and /TH/. In English it was eventually superseded by the digraph th. Compare with eth.

  [so named from the word of which it was the first letter]