[AIVC, Air infiltration and ventilation glossary, 1992]:an expression that describes the air leakage rate of a building or component. This may be: a) The air leakage flow rate at a reference pressure difference across the component or building envelope, (Usually 50 Pa and expressed in terms of C (the Flow Coefficient) and n (the Flow Exponent). b) The relationship between the flow rate and the pressure difference across the building envelope or component. c) The equivalent leakage area (ELA) at a reference pressure difference across the component or building envelope.(see flow equation; equivalent leakage area).