cosigner vs signer

cosigner and signer both are nouns.

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

As nouns, signer is a hypernym of cosigner; that is, signer is a word with a broader meaning than cosigner:

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

Other hypernyms of cosigner include signatory.

As nouns, signer is a hypernym of cosigner; that is, signer is a word with a broader meaning than cosigner:

cosigner: a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)
signer: someone who signs and is bound by a document

Other hypernyms of cosigner include signatory.

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