Fault Domain

In Azure a Fault Domain means a rack of servers. It consumes network, power and cooling; 2 VMs in the same availability set means Azure will provision them in to 2 different racks.

There are always 2 or 3 fault domains: FD0, FD1 and FD2

Source:

https://blogs.msdn.microsoft.com/plankytronixx/2015/05/01/azure-exam-prep-fault-domains-and-update-domains/

https://docs.microsoft.com/en-us/azure/virtual-machines/windows/manage-availability