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 refer to physically separate locations within a larger geographic area used to design for high availability. AWS and Azure call them Availability Zones; Google Cloud uses Zones within a Region; OCI uses Availability Domains (and also Fault Domains for additional separation within an AD).