• Guarantee – In law and common usage: to undertake or engage for the payment of (a debt) or the performance of (a duty) by another person; to undertake to secure (a possession, right, claim, etc.) to another against a specified contingency, or at all avents; to give a guarantee concerning; to engage, assure, or secure as a thing that may be depended on; to warrant; as, to guarantee the execution of a treaty.