high availability cluster (HA cluster)

A high availability cluster is a group of hosts that act like a single system and provide continuous uptime. 

High availability clusters are often used for load balancing, backup and failover purposes. To properly configure a high-availability (HA) cluster, the hosts in the cluster must all have access to the same shared storage. This allows virtual machines (VMs) on a given host to fail over to another host without any downtime in the event of a failure.

HA clusters can range from two nodes to dozens of nodes, but storage administrators must be wary of the number of VMs and hosts they add to an HA cluster because too many can complicate load balancing.