An airtight closure is a type of SEAL that is designed to prevent air or other gases from entering or exiting an enclosed space. It is often used in containers, such as food storage containers, to keep the contents fresh and free from contamination. The term 'airtight closure' can also refer to the process of sealing a container or other space with a material that is impermeable to air. The crossword solution for this term is 'SEAL, ' which is a four-letter word.