The Collaborative International Dictionary of English v.0.48:
Onomastic \On`o*mas"tic\, a. [Gr. ?, from ? to name, 'o`noma name.] (Law) Applied to a signature when the body of the instrument is in another's handwriting. --Burrill. [1913 Webster]WordNet (r) 3.0 (2006):
onomastic adj 1: of or related to onomastics; "he published a collection of his onomastic essays"