LPAR

LPAR stands for logical partition. It lets a single physical mainframe be divided into several separate computers, each with its own share of processors, memory and storage, and each running its own copy of an operating system.

The partitions are properly isolated. A program running in one LPAR cannot see or disturb what is happening in another, even though they share the same physical machine. That isolation is strong enough that organisations routinely run their production systems and their test systems as different LPARs on the same hardware.

This is why an organisation with one mainframe might talk about having a development system, a test system and a production system. They are usually LPARs, not separate machines.

If you have used virtual machines on a laptop or in the cloud, the idea will be familiar. LPARs are the same concept, and mainframes have had them since the 1960s. The practical thing to remember is that when a colleague says "which system are you on?", they usually mean which LPAR.