The Dorabella Cipher is an enciphered letter written by Edward Elgar to Miss Dora Penny, which was accompanied by another dated July 14, 1897. Penny was never able to decipher it and its meaning remains unknown.
The cipher, consisting of 87 characters spread over 3 lines, appears to be made up from 24 symbols, each symbol consisting of 1, 2, or 3 approximate semicircles oriented in one of 8 directions (the orientation of several characters is ambiguous). A small dot appears after the fifth character on the third line.
A count of the 87 characters reveals a symbol frequency very close to plain text in English, but attempts to decipher it as a simple substitution cipher have so far proved fruitless.
The longer Wikipedia page, which lists several attempted solutions, is here. The Cipher Mysteries page is here, the Cipher Foundation page is here. There is various other coverage.
See also Enigma Variations