Others have answered, but I think the general principle for a Region is that between the AZs you have less than 1ms latency, physical separation between the data centers, but they may still be in the same floodplain, be able to be hit by the same hurricane, etc.
More info here if you want to see how regions are structured at a high level of detail: https://youtu.be/AyOAjFNPAbA?t=15m21s