Also known as: OS/400, i5/OS
The operating system descended from the original AS400's OS/400, now running on current IBM Power Systems hardware, with an integrated relational database (DB2 for i) built into the OS itself.
IBM i is the operating system that traces directly back to the AS400's original OS/400, renamed i5/OS and later IBM i as the underlying hardware moved from AS/400 to eServer iSeries, System i, and now Power Systems. What made it distinctive from the start, and still does, is that the relational database (DB2 for i) is built into the operating system itself rather than bolted on as a separate product, along with object-based security and single-level storage.
IBM i is one of three operating systems IBM Power Systems hardware can run, alongside AIX (IBM's Unix) and Linux. A business migrating "AS400 to Power Systems" is almost always moving an existing IBM i workload onto new Power hardware, not switching operating systems.