Using two or more computer systems that work together. It generally refers to multiple servers that are linked together in order to handle variable workloads or to provide continued operation in the event one fails. Each computer is a multiprocessor system itself. For example, a cluster of four computers, each with 16 CPU cores, would enable 64 unique processing threads to take place simultaneously.
A cluster of servers provides fault tolerance and/or load balancing. If one server fails, one or more servers are still available. Load balancing distributes the workload over multiple systems.