[ASHRAE 62.1]:
any indoor area that requires ventilation and comprises one or more spaces with the same occupancy category (see Table 6.2.2.1), occupant density, zone air distribution effectiveness (see Section 6.2.2.2), and design zone primary airflow (see Section 6.2.5.1) per unit area

Informative Note: A ventilation zone is not necessarily an independent thermal control zone; however, spaces that can be combined for load calculation purposes can often be combined into a single zone for ventilation calculations purposes.

[EN 16798-7:2017, 3.2]:
conditioned space which can be considered as air flow independent from other conditioned spaces (i.e. the air leakages between two adjacent zones are sufficiently low to be neglected, and there is no possibility of air transfer between two zones)Note 1 to entry: A ventilation zone can be a set of rooms with enough air transfer between those rooms to consider they are all at the same pressure. It can also be a single room if there is no significant air transfer between this room and other ventilation zones.