A promise by one party (the guarantor) to pay off a loan in place of the borrower; often demanded by the lender as a condition of providing a loan. Also called "co-signing".
A loan guarantee (also called co-signing) is a promise by one party (the guarantor) to pay off a loan instead of (or together with) the borrower.
Institutional lenders such as banks often require a loan guarantee for granting a loan or mortgage, to protect them from default of the borrower. A loan guarantee is a type of surety.