z/OS is the computer operating system for IBM's zSeries 900 (z900) line of large (mainframe) servers. z/OS is a renamed and upgraded version of OS/390, which in turn evolved from the MVS operating system. IBM's renamed servers and operating systems reflect a strategy to realign its products more closely with the Internet and its own e-business initiatives.
z/OS is described as an extremely scalable and secure high-performance operating system based on the 64-bit z/Architecture. Like its predecessor, OS/390, z/OS lays claim to being highly reliable for running mission-critical applications. The operating system supports Web- and Java-based applications.