Ullage is the term used to describe the empty space in a container, such as a bottle or a tank. In the context of a fuel storage tank, ullage refers to the empty space at the top of the tank above the fuel level. This space is necessary to allow for expansion of the fuel as it warms up and for the movement of the fuel as it is pumped out of the tank. The ullage in a fuel tank is typically measured in inches or centimeters, and it is important to monitor the ullage level to ensure that the tank does not run out of fuel or that fuel is not lost due to overflow.