The English letter historically called thorn is Þ, written in lowercase as þ.
Thorn was used in Old English and Middle English to represent the sounds now written with “th,” including both the sound in “thin” and the sound in “this.” It originated from the runic character ᚦ, associated with the Elder Futhark rune called thurisaz, and was adapted into the Latin-based writing system used for Old English.
Thorn gradually declined as the digraph “th” became more common, especially during the Middle English period. Printing accelerated its disappearance because imported type often lacked a thorn character. This contributed to historical spellings such as “ye olde,” where the “y” visually substituted for a thorn; the word was still intended to mean “the,” not pronounced “yee.”
Thorn is not the same as eth, ð, although both letters could represent similar sounds in Old English. Thorn survives as a regular letter in modern Icelandic, where it remains part of the alphabet.