Which term refers to the part of the coffin that provides an airtight seal?

The term that refers to the part of the coffin that provides an airtight seal is the sealing gasket. A sealing gasket is specifically designed to create a tight, air-tight barrier when the lid of the coffin is closed, helping to preserve the contents by preventing the entry of moisture and gases, thus contributing to the longevity of the remains.

Other options include the plastic flange, which is generally a structural component that may relate to the overall design but does not specifically function as a seal, and the closure strip, typically used for securing the closure mechanism but also not designed to create an airtight environment. The term "hermetic seal" indicates a complete absence of air, which is more of a general term that can encompass various sealing methods, but in the context of coffins, the term sealing gasket is the most accurate as it directly describes the part that reliably achieves the airtight seal necessary for a coffin.