To assert the truth of (something); to affirm (something) with confidence; to declare (something) in a positive manner.
To justify or prove (an allegation or plea that one has made).
To give an assurance that something will be done right.
To make something certain.
To assume or take responsibility for a debt or other obligation.
The person to whom a guarantee is made.
A person who gives such a guarantee; a guarantor.
A legal assurance of something, e.g. a security for the fulfillment of an obligation.
More specifically, a written declaration that a certain product will be fit for a purpose and work correctly; a warranty
Anything that assures a certain outcome.