Availability Set
A grouping of VMs across fault and update domains within a datacenter to improve uptime.
An availability set groups virtual machines within a single datacenter across two dimensions: fault domains (separate physical racks with independent power and networking, so a hardware failure affects only one group) and update domains (staggered sets that Azure reboots one at a time during planned maintenance, keeping the others running). Together they protect against both unplanned hardware outages and planned maintenance. The exam often tests sets versus availability zones: sets guard against failures inside one datacenter, while zones spread VMs across physically separate facilities within a region. For a datacenter-level outage, zones are the right answer, not sets.
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