Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

allograph
     n 1: a variant form of a grapheme as `m' or `M'
     2: a signature made by one person for another