guarantee
noun.
a written assurance that some product or service will be provided or will meet certain specifications
noun.
an unconditional commitment that something will happen or that something is true
there is no guarantee that they are not lying
noun.
a collateral agreement to answer for the debt of another in case that person defaults
Synonym: guaranty
verb.
give surety or assume responsibility
I vouch for the quality of my products
Synonym: vouch
verb.
make certain of
This nest egg will ensure a nice retirement for us|Preparation will guarantee success!
verb.
promise to do or accomplish
guarantee to free the prisoners
Synonym: undertake
verb.
stand behind and guarantee the quality, accuracy, or condition of
The dealer warrants all the cars he sells|I warrant this information
Synonym: warrant