IBM S390, now branded System z, is a line of "big iron" mainframe servers.

IBMs S/390 (System/390) brand was introduced in the early 1990s as the successor to the aging System/370 line of mainframes that had been around since the 1970s and before that the S/360. Based on the Enterprise Systems Architecture/390 which was announced in 1990, it would be the final line of 32bit mainframes before transitioning to 64bit in the year 2000. S/390s continued the trend of backwards compatibility with systems dating back to 1964.

IBM S/390 (System z) homepage