A computer cluster is a group of linked computers, working together closely thus in many respects forming a single computer. The components of a cluster are commonly, but not always, connected to each other through fast local area networks. Clusters are usually deployed to improve performance and availability over that of a single computer, while typically being much more cost-effective than single computers of comparable speed or availability.
And now I will put together clusters for high availability and load balancing. This is where VMs come in handy. I think I prefer loosely coupled clusters, clusters where individual nodes are separated on different power grids.
It’s always nice to have a succinct definition handy to stay focused.