Isolated data centers within a cloud region that provide backup and redundancy. Like having multiple bank branches in the same city - if one has problems, others keep working.
AWS regions typically have 3-6 availability zones, so if one zone has a power outage, your application continues running in another zone.
All four describe physically separate locations within a broader geography used for high availability. AWS/Azure use “Availability Zone” within a Region; GCP uses “Zone” within a Region; OCI uses “Availability Domain” within a Region (and also has “Fault Domains” for additional isolation inside an AD).