A measure of how tight a valve
can be closed. When one side of the valve is pressurized
with air, not even a bubble passes through to the other side.
This is usually achieved by means of a soft valve seat using Teflon, EPDM or some other elastomer. However, it can be achieved with metal to metal seating provided that the components are lapped together for some time.
As the components in a valve wear, bubble-tightness will be lost at which point the valve is either repaired or replaced.