Source : Webster's Revised Unabridged Dictionary (1913)

Onomastic \On`o*mas"tic\, a. [Gr. ?, from ? to name, ? name.]
   (Law)
   Applied to a signature when the body of the instrument is in
   another's handwriting. --Burrill.

Source : WordNet®

onomastic
     adj : of or related to onomastics; "he published a collection of
           his onomastic essays"