What is an equivalent term (in English) for collection (artificially or physically) machines?

Currently there are not many sites that are called server farms.

Some servers are called server clusters. Usually all of this is done by the owner. A server farm isn’t a server farm. It is a server. Server farms are where thousands of computers running at the same time consume heavy power to run and to keep cool.

Server farms are small cluster applications that run the software that allows your most on-premises applications (such as the stackengine network; Gmail, etc.).

Is a cluster a way to write a system, and not a series of machines, where 1 or 2 individual processors is part of a distributed network? It should not be confused with a server farm in that a server farm can run many different applications on the same set of hardware.

Distributed computing is an adjective that describes tasks that are run over remote machines and are usually clustered (as is the case with SETI@home).

How one could refer to process of outsourcing a task to a set of machines?

In computer science specifically, tasks are offloaded onto remote computing resources:

In computer science, computation offloading refers to the transfer of certain computing tasks to an external platform, such as a cluster, grid, or cloud.

Definitions taken from Wikipedia.

(Definitions taken from Wikipedia)