This noun means a guarantee, surety, or bondsman—one who pledges to ensure another's obligation is fulfilled. Hebrews 7:22 declares Jesus the 'guarantee of a better covenant,' meaning He personally ensures all covenant promises will be kept. Classical Greek used it for someone providing financial security or bail. Unlike the old covenant depending on human faithfulness (which failed), the new covenant rests on Jesus' perfect guarantee—His own person and work secure what He promises. He is both promise-maker and promise-keeper.