guarantee

1. Something that assures a particular outcome or condition: Lack of interest is a guarantee of failure.
2. 1. A promise or an assurance, especially one given in writing, that attests to the quality or durability of a product or service.
   2. A pledge that something will be performed in a specified manner.
3. 1. A guaranty by which one person assumes responsibility for paying another's debts or fulfilling another's responsibilities.
   2. A guaranty for the execution, completion, or existence of something.
4. A guarantor.

1. To assume responsibility for the debt, default, or miscarriage of.
2. To assume responsibility for the quality or performance of: guarantee a product.
3. To undertake to do, accomplish, or ensure (something) for another: guaranteed to free the captives; guarantees freedom of speech.
4. To make certain: The rain guarantees a good crop this year.
5. To furnish security for.
6. To express or declare with conviction: I guarantee that you'll like this book.

[Alteration of Middle English garant, warranty, from Old French. See guaranty.]