Availability Zone
In plain terms
An isolated datacenter location within a cloud region.
Going deeper
Availability Zones are physically separate datacenters in a region, so spreading across them survives a single facility's failure. They enable high availability.
Under the hood
Each AZ has independent power/cooling/network; multi-AZ deployment is a baseline HA practice.
Examples
Multi-AZ deployments, high availability, fault tolerance, regional redundancy.
Think of it like...
Like keeping copies of your work in separate buildings so one fire can't destroy all.