What is an AWS availability zone (AZ)?

An availability zone is an isolated set of resources within a region. Each region has at least 2 az. The idea is that a fire or power outage in one AZ should not affect another AZ. Each AZ is also connected via a high-speed network connection. This allows the architect to run as if the accessor is in the same data centre, but with the benefit that if something happens in one AZ it usually does not affect other AZs in the region.