seal
Definitions
A device or substance used to join two things together so as to prevent leakage or passage.
A marine mammal with flippers, a sleek body, and blubber, typically found in cold seas.
To fasten or close something securely to prevent access or the escape of contents.
Examples
The zookeeper watched the seal swim gracefully in the large pool.
Make sure to seal the envelope tightly before you drop it in the mailbox.