To close or fasten something so as to make it airtight, watertight, or otherwise secure against leakage or intrusion. She sealed the envelope before dropping it in the mailbox.
To formally confirm or ratify (a document, agreement, or commitment), often by affixing a seal or other mark of authority. The two companies sealed the deal with a signed contract.
To make something irrevocable or decide the outcome of something, ensuring a particular result. Her late-game goal sealed the team's victory.