An "airtight seal" means that something is closed or covered so tightly that no air can get in or out. It is used to keep things fresh, prevent leaks, or maintain a controlled environment. Full definition
Look for jars with airtight seals to keep your goodies fresh. (bhg.com)
I use swing - top clear glass bottles as they are easier for pouring, maintain a nice airtight seal, and I can see what is going on. (userealbutter.com)
The heavy - weight lid offers a secure airtight seal that prevents air from getting into the chamber after shutting the lid. (VacuumMania.com)