密封 (mì fēng): to seal up

After opening this jar of jam, you need to put it in the fridge and keep it sealed, otherwise it will go bad easily.

Please seal the envelope properly, then put a stamp on it and send it out.

The worker sealed the opening of the cardboard box with tape to prevent the things inside from falling out.

This medicine must be stored sealed; once it gets damp, it can no longer be taken.

To prevent air from getting into the pipe, the engineer sealed the joint very tightly.