The term 'completely airtight' can be described by four crossword solutions: HERMETIC (8 letters), ALIBI (5 letters), SEAL (4 letters), and seals (5 letters). HERMETIC refers to something that is completely sealed off from the outside environment, while an ALIBI is an excuse or explanation used to defend oneself from a charge of wrongdoing. A SEAL is a device used to close off an opening, and seals are animals that live in the ocean and use their flippers to move around. Together, these words describe something that is completely airtight and impermeable.