Definitions of guarantee:

• noun:   a written assurance that some product or service will be provided or will meet certain specifications
• noun:   a pledge that something will happen or that something is true

  Example: "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
• verb:   give surety or assume responsibility
• verb:   make certain of

  Example: "Preparation will guarantee success!"

• verb:   stand behind and guarantee the quality, accuracy, or condition of
• verb:   promise to do or accomplish

  Example: "Guarantee to free the prisoners"