Nouns

(n)guarantee, warrant, warrantee, warrantya written assurance that some product or service will be provided or will meet certain specifications
(n)guaranteean unconditional commitment that something will happen or that something is true“there is no guarantee that they are not lying”
(n)guarantee, guarantya collateral agreement to answer for the debt of another in case that person defaults

Verbs

(v)guarantee, vouchgive surety or assume responsibility“I vouch for the quality of my products”
(v)guarantee, ensure, insure, assure, securemake certain of“This nest egg will ensure a nice retirement for us”, “Preparation will guarantee success!”
(v)undertake, guaranteepromise to do or accomplish“guarantee to free the prisoners”
(v)guarantee, warrantstand behind and guarantee the quality, accuracy, or condition of“The dealer warrants all the cars he sells”, “I warrant this information”