What does the n50 value mean?

n₅₀ is a number used to quantify building airtightness. It gives the air change rate occurring as a result of air leakage through the building envelope when an air pressure difference of 50 Pa exists between inside and outside. This air change rate is the leakage volume airflow rate in m³/h measured at 50 Pa pressure difference divided by the buildings’ internal volume. It is expressed in h^-1.