guarantee

a written assurance that some product or service will be provided or will meet certain specifications

an unconditional commitment that something will happen or that something is true; "there is no guarantee that they are not lying"

a collateral agreement to answer for the debt of another in case that person defaults

give surety or assume responsibility; "I vouch for the quality of my products"

make certain of; "This nest egg will ensure a nice retirement for us"; "Preparation will guarantee success!"

promise to do or accomplish; "guarantee to free the prisoners"

stand behind and guarantee the quality, accuracy, or condition of; "The dealer warrants all the cars he sells"; "I warrant this information"