Names
From MidrangeWiki
The AS/400 / iSeries / i5 has had a number of names over the years.
It started life as the Application System/400 ... or AS/400.
It combined the best of S/38 and S/36, leaning on S/38 for OS foundation and S/36 for user-friendliness. Many IBM customers on S/36 SSP were slow to embrace the AS/400 so IBM introduced the Advanced System 36 ... or AS/36 which had several models such as AS/236 and AS/436 with upgrade paths to AS/400 models.
Around the year 2000, IBM changed it's name to eServer iSeries (The "i" indicating "Integrated"?)
Then, in 2004, the name was changed to i5. The "5" referring to the Power5 Processor that was at the heart of the system. At the same time OS/400 was renamed to i5/OS.
Confused? I sure am.