Availability Zone
also called AZ
One or more physically separate data centres inside a cloud region, with independent power, cooling and network, connected by low-latency links.
The zone is the unit of correlated physical failure in a cloud region. Spreading across zones is the single highest-value reliability decision available, because the latency cost is around a millisecond and the failure modes it removes — a power event, a cooling failure, a fire — are real and regular.
Multi-zone is not multi-region. Zones share a region's control plane, and regional service outages have taken out all zones simultaneously more than once. If the requirement is to survive a region, that is a much larger and much more expensive design, with real data-consistency consequences.
Charging note: inter-zone data transfer usually costs money. A chatty multi-zone architecture can produce a surprising line item.