See zSeries.

A family of IBM mainframes introduced in 2000. It uses the z/Architecture, a major upgrade to the ESA/390 architecture. The zSeries is a departure in naming for IBM, which had used the System/360, 370 and 390 designations for decades. The first zSeries models were the high-end 900 line, followed in 2002 by the zSeries 800, geared to the mid-size customer. All IBM products come under the eServer branding, thus a mainframe is a zSeries eServer. See z/OS, z/Architecture, IBM server series and System/390.