Meaning – The term OS/400, refers to the IBM operating system for the AS/400 system. It was originally released in 1988 as OS/400, alongside the IBM AS/400 line of systems. It was renamed to i5/OS in 2004, before being renamed a second time to IBM I in 2008. It is an evolution of the System/38 CPF operating system, with compatibility layers for System/36 SSP and AIX applications.

The AS/400 product line was rebranded multiple times throughout the 1990s and 2000s. As part of the 2004 rebranding to eServer i5, OS/400 was renamed to i5/OS; the 5signifying the use of POWER5 processors. The first release of i5/OS, V5R3, was described by IBM as “a different name for the same operating system”.

Example of usage“In 2006, IBM rebranded the AS/400 line one last time to System i. In April 2008, IBM consolidated System I with the System p platform to create IBM Power Systems. At the same time, i5/OS was renamed to IBM I, in order to remove the association with POWER5 processors.”