seal, seals, sealed, sealing, unsealed, signet ring, signet

Definition:

To seal an object means to keep it closed with something that makes it impossible to open without breaking the seal.

Often a seal is marked with a design to show who it belongs to.
Melted wax was used to seal letters or other documents that needed to be protected. When the wax cooled and hardened, the letter could not be opened without breaking the wax seal. The person who received the letter would see the unbroken seal and know that no one had opened it.
A seal was put on the stone in front of Jesus' grave in order to keep anyone from moving the stone.
Paul figuratively refers to the Holy Spirit as a "seal" showing that our salvation is secure.