Let's not take the definition of cluster for granted. In essence, a cluster is a collection of networked computing resources and services, collapsed into a logical entity that masquerades as a single computer system. When we say cluster, in typical human fashion, we collapse a lot of complexity into a single word. This is similar to a user who says, "The network is sure slow today."
I use the following formal definition for cluster:
- A closely coupled, scalable collection of interconnected computer systems, sharing common hardware and software infrastructure, providing a parallel set of resources to services or applications, for improved performance, throughput, or availability.
This formal definition still doesn't tell us how the word is being used by a particular group of people. Remember that the word cluster has an overloaded meaning. Saying cluster to a scientist will trigger a different picture and set of expectations than if you mention the term to an IT person. Make sure that your audience understands and agrees with your use of the word.