Operating system of IBM AS/400 mainframes (called "minicomputers", not "mainframes", by some). Transaction-oriented (that is, designed for lots of I/O but unchallenging computation; RPG is a "native language"); nothing to do with AIX.
Tcl/Tk has been ported there: [1 ]
In 2000, IBM changed the branding (again!). The hardware is now "eServer", and the "platform" is "iSeries" [4 ]. So no one writes of AS/400; from here on, it's iSeries. It's not consistent, at least to engineers; IBM still needs to say "OS/400" occasionally, because in fact the company makes available implementations of Linux, AIX, and Windows2000, along with OS/400, to run on iSeries hosts.
It changed again. In 2004, "i5/OS" succeeded OS/400. Instead of calling them "iSeries", they're now "eServer i5"-s, to emphasize that the identical hardware hosts Linux and AIX, sometimes simultaneously.