Availability Set is a grouping of Azure Virtual Machines. The availability set allows the Azure cloud to build and understand how the application for a user is constructed to provide availability and redundancy.

An Availability Set in Microsoft Azure is a logical grouping of two or more virtual machines (VMs) within the same data center. The purpose of an Availability Set is to provide high availability to your applications, ensuring that they remain operational even if one of the VMs within the set becomes unavailable due to planned or unplanned maintenance events.

Key characteristics of an Availability Set include:

Fault Domain: VMs in an Availability Set are automatically distributed across different physical hardware (known as fault domains). This helps protect your application from hardware failures or planned maintenance events that might affect a specific piece of hardware.
Update Domain: VMs in an Availability Set are also distributed across different logical groupings called update domains. During planned maintenance, Azure updates one update domain at a time, ensuring that a subset of VMs within the Availability Set is always available.

By using an Availability Set, you can improve the reliability and availability of your applications running in Azure, making them more resilient to failures and maintenance activities. It’s an essential concept for designing highly available and fault-tolerant solutions on the Azure platform.