allograph

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A variant shape of a letter.
2. n. A letter or combination of letters that can represent one phoneme, as f and gh can represent the phoneme /f/.
3. n. Writing, especially a signature, made by one person for another.

Century Dictionary and Cyclopedia

1. n. In law, a deed not written by any of the parties to its execution: opposed to autograph.

Wiktionary

1. n. A variant form of letter (or other grapheme)
2. n. A signature made by one person for another (compare autograph)

GNU Webster's 1913

1. n. A writing or signature made by some person other than any of the parties thereto; -- opposed to autograph.

WordNet 3.0

1. n. a signature written by one person for another
2. n. a variant form of grapheme, as `m' or `M' or a handwritten version of that grapheme