Definition from Nolo’s Plain-English Law Dictionary
When used as a verb, to agree to pay another person's debt or perform another person's duty, if that person fails to come through. As a noun, the written document in which this assurance is made. For example, if you cosign a loan, you have made a guaranty and will be legally responsible for the debt if the borrower fails to repay the money as promised. The person who makes a guaranty is called the guarantor. Also known as a guarantee or warranty.