[AIVC, Air infiltration and ventilation glossary, 1992]:
the volumetric rate at which air enters (or leaves) a building or zone expressed in units of building or zone volume(see specific flow)

[ASHRAE Standard 62.2]:
airflow in volume units per hour divided by the volume of the space on which the air change rate is based in identical units (normally expressed in air changes per hour

[ach]).

[ISO 16814:2008, 3.5]:
air flow rate to a space, expressed as volume per unit time, divided by the volume of the space in consistent units.NOTE Air change rate is often expressed as air changes per hour.