A container or barrier that is hermetic is sealed so that not even air can enter or leave. For example a tin (or can).

Etymology

This word comes from the Greek god and mythological alchemist Hermes Trismegistus. He was known to possess a magic ability to seal (with spells) treasure chests so that nothing could access its contents. Alchemists also frequently used distillation in their experiments, and needed an airtight seal to improve the efficiency of their alembic stills.

See also Hermeticism.