These two terms are commonly used, essentially they mean the same thing but in different ways – “Airtight” is the state of being impermeable to air or other gases; whereas “Hermetic” refers to a method of making something airtight, so for example a door with “Hermetic seals” will be airtight. This is also commonly referred to as “Air Permeability” – essentially the amount of air that can pass through the door. There are different standards of airtightness which is usually measured in terms of leakage per square meter per hour at a certain pressure (usually 50 Pascals), so for example 10 M3 / M2 / Hr @ 50 Pa. Airtightness in doors is measured/tested in accordance with BS EN 12426, reported in terms of Classes 0 to 6 (6 is the lowest air permeability). Sometimes doorsets will be tested to achieve a certain airtightness and sometimes it will be based on a calculation, for example by adding extra seals to a standard doorset so be sure to understand what it is you need.