Definition
An LPAR, or logical partition, is a virtual division of a physical Power server that behaves like its own independent machine, with its own memory, processor allocation, and operating system instance. Most IBM i shops already run production on an LPAR rather than a whole physical box, often alongside other partitions running AIX, Linux, or additional IBM i instances on the same hardware.
LPARs matter directly in a hardware refresh. When a Power 9 system reaches end of life, the migration is really a partition migration: the LPAR's configuration, activated cores, memory allocation, and attached storage all need to be re-created or moved on the new Power 10 or Power 11 hardware. The IBM i software running inside that partition does not need to change, but the partition definition itself is part of the migration plan.