(SYStem comPLEX) The multiprocessing capability in IBM MVS/ESA and OS/390 mainframes. Introduced with MVS/ESA 4.3 in 1991, it enables the linking of multiple system images within multiple machines and treating them all as a single image. Sysplex added the external Sysplex Timer to synchronize internal clocks. If a failure occurs in a multiprocessor complex, precise transaction time stamps are required for accurate rollback and recovery. It also added XCF (Cross-system Communications Facility), which is software that manages the interconnections between machines.

The addition of one or more Coupling Facilities creates a Parallel Sysplex system that provides high-level clustering that includes data sharing with guaranteed integrity and extensive resource sharing between the systems. See Coupling Facility and Parallel Sysplex.