signer vs cosigner

signer and cosigner both are nouns.

Word	Noun	Adjective	Verb	Adverb
signer	Yes	No	No	No
cosigner	Yes	No	No	No

As nouns, cosigner is a hyponym of signer; that is, cosigner is a word with a more specific, narrower meaning than signer:

signer: someone who signs and is bound by a document
cosigner: one of two or more signers of the same document (as a treaty or declaration)

Other hyponyms of signer include cosignatory, abhorrer, endorser, indorser, attestant, attestator, attestor, witness.

signer (noun)	cosigner (noun)
someone who signs and is bound by a document	one of two or more signers of the same document (as a treaty or declaration)
someone who can use sign language to communicate	a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)