guarantee: "a formal assurance that certain conditions will be fulfilled, especially that a product will be of a specified quality" warranty: "a written guarantee promising to repair or replace an article if necessary within a specified period." .. A warranty is a promise or guarantee given. A warranty is usually a written guarantee for a product and declares the maker's responsibility to repair or replace a defective product or its parts. A guarantee is an agreement assuming responsibility to perform, execute, or complete something and offering security for that agreement. It is a promise or an assurance, especially one given in writing, that attests to the quality or durability of a product or service, or a pledge that something will be performed in a specified manner.