Something that assures a particular outcome or condition: Lack of interest is a guarantee of failure. A promise or assurance, especially one given in writing, that attests to the quality or durability of a product or service. A pledge that something will be performed in a specified manner. A guarantor. A guaranty. To assume responsibility for the debt, default, or miscarriage of. To assume responsibility for the quality or performance of: guarantee a product. To undertake to do, accomplish, or ensure (something) for another: guaranteed to free the captives; guarantees freedom of speech. To make certain: The rain guarantees a good crop this year. To guaranty. To furnish security for. To express or declare with conviction: I guarantee that you'll like this book.