In a double wall tank, the interstice is the space between the inner tank and the outer tank.
Interstice can also refer to the space between two containment walls in an Under Dispenser Containment (UDC) sump, or any other sump other than a tank.
It is this space that can be monitored for leaks in the walls of either the inner or outer tanks. In double-wall piping, the space between the inner and outer pipes is also referred to as the interstice.
Another, less-used term for the interstice, is annulus. Annulus literally means "ring."