It is one of the operating systems supported on IBM Power Systems alongside AIX and Linux as well as on IBM PureSystems alongside AIX, Linux and Windows. source
V7R1,2,3 is a sucessor of OS/400.
A fact on the name;
When IBM announced the new Power Systems line of servers on April 2,
2008, they renamed the operating system from i5/OS to IBM i and
changed the version identifier format from VxRxMx (Version, Release,
Modification, e.g. V6R1M0) to the more standard format (e.g. 6.1).