Sealed so completely that air can neither enter nor escape, a property that underpins much of modern food preservation. Airtight packaging protects foods from oxygen that would cause rancidity and staleness, from moisture that would soften crisp products or encourage mold, and from the aromas of neighboring foods that unsealed items readily absorb. Canning achieves airtightness with hermetic double seams, while jars, pouches, and bottles rely on gaskets, heat seals, and tamper-evident closures. The integrity of the seal is a food safety matter: a compromised airtight container can admit microbes or lose the vacuum that signals safety to the consumer. Testing ranges from simple vacuum checks to helium leak detection on high-speed lines, because a seal is only as good as its weakest millimeter.