(Logical PARtition) A logical segmentation of a mainframe's memory and other resources that allows it to run its own copy of the operating system and associated applications. LPARs are caused by special hardware circuits and allow multiple system images to run in one machine. This can be multiple instances of the same operating system or different operating systems. In the IBM world, this is known as a "virtualized processor complex."

IBM's PR/SM ("prism"), Hitachi's MLPF and Amdahl's MDF are examples of LPARs. MDF was the first to provide in hardware the equivalent of IBM's VM operating system, which supports multiple system images in software. Interestingly, an LPAR can host VM, which itself can host multiple operating systems. See PR/SM and virtual machine.