Synonyms for Guarantee (same or very similar meaning)
WordNet sense 1 (a collateral agreement to answer for the debt of another in case that person defaults):
guaranty
WordNet sense 2 (a written assurance that some product or service will be provided or will meet certain specifications):
warrant, warranty, warrantee
WordNet sense 3 (make certain of):
assure, ensure, insure, secure
WordNet sense 4 (give surety or assume responsibility):
vouch
WordNet sense 5 (promise to do or accomplish):
undertake
From the ODE community, based on WordNet
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