The Collaborative International Dictionary of English v.0.48:

Allograph \Al"lo*graph\, n. [Gr. ? another + -graph.]
   A writing or signature made by some person other than any of
   the parties thereto; -- opposed to autograph.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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