"cosigner" in the noun sense
1. cosigner, cosignatory
one of two or more signers of the same document (as a treaty or declaration)
a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)
Source: WordNet® (An amazing lexical database of English)
Princeton University "About WordNet®."
WordNet®. Princeton University. 2010.
View WordNet® License