Completely sealed is a term that denotes something that is entirely closed or impervious to any external factors. Synonyms for this term include fully enclosed, hermetically sealed, airtight, watertight, airproof, leak-proof, sealed shut, tightly shut, and lock-tight. Each of these words describes the state of an object, container, or system that is closed off completely, leaving no room for any intrinsic or extrinsic factors to enter or escape. Whether it's a package, container, or a building, using any of these synonyms to mean "completely sealed" will ensure that there's no ambiguity about the object's state of closure or protection.