• A security or pledge evidenced by a promise by one party to pay a debt or perform an obligation contracted by another in the event the original obligor fails to pay or perform as contracted.

    Internal Revenue Service 1

1Internal Revenue Service, Internal Revenue Manual 4.42.6 Glossary, http://­­irm/­part4/­irm_04-042-006.html (last accessed Dec. 22, 2009).