密封 (mì fēng): Seal (eg. airtight (Verb) — Meaning & Stroke Order

To preserve the freshness of the coffee beans, they must be sealed in a light-proof jar.

Are you sure this package is tightly sealed and won't be damaged during transport?

These documents need to be sealed for storage to prevent leakage.

Due to not being sealed in time, that batch of experimental samples has already been contaminated.