A co-signer is a person who signs a contract or agreement along with another person, typically to guarantee payment or assume responsibility. Other terms that are commonly used for co-signer include guarantor, co-borrower, co-applicant, and joint applicant. A guarantor is someone who promises to pay off the debt or loan in case the borrower defaults, while a co-borrower or co-applicant is someone who is equally responsible for paying off the debt. A joint applicant, on the other hand, shares equal ownership or liability for the loan or agreement. These terms are often used interchangeably, depending on the specific context and legal requirements of the agreement.