When the machine (a combination of the S/38 and S/36) came out, it was called the AS/400 or Application System/400, which was widely agreed to be a terrible name for marketing purposes.
Several years ago, IBM decided to rename its hardware architectures. The AS/400 was renamed the iSeries, the mainframe (S/390) was called the zSeries and their UNIX box became the pSeries.
More recently, the iSeries was renamed again as the eSeries i5.
The term AS/400 is still widely used to refer to the machine by users who have worked on it for many years.