Spreading the work load of a task over several servers in a fashion that the user doesn't notice. An example would be a high traffic website spreading the requests over a cluster of application servers.

This can be achieved through the use of commercial products like CiscoDirector or OpenSource free products like LinuxDirector? (part of the LinuxVirtualServer? project) or ModBackhand.

They mostly work at different levels of the ISO model (ie, TCP connections or HTTP requests) and all have disadvantages and advantages.

Commonly combined with HighAvailability.