a seal: the stamped mark, or the signet that makes it, used to keep something closed, mark whose it is, and vouch that it is genuine
A seal in the ancient world was a small engraved signet, often a ring or a carved stone, pressed into soft wax or clay to leave a mark no one else could copy. That single object did several jobs at once, and this word covers both the signet and the impression it left behind.
The uses in the New Testament trace those jobs. A sealed scroll could not be opened without breaking the seal, so in Revelation the seven seals both hold the scroll shut and guard its contents until the one worthy to open it breaks them. A seal stamped on a person marked ownership: the servants of God carry his seal on their foreheads as a sign that they belong to him and are under his care, standing opposite the χάραγμα, the mark of the beast.
From there the word stretches naturally to anything that confirms or guarantees. Paul calls circumcision a seal on the righteousness Abraham already had by faith, and calls the Corinthian believers the seal that proves his commission is real. In each case the seal is what certifies that something is genuine and authorized. The matching verb (σφραγίζω) names the act of stamping such a mark.