Noun
sysplex (plural sysplexes)
(computing) A group of connected IBM mainframe systems that share resources.
Another central design philosophy is support for extremely high quality of service (QoS), even within a single operating system instance, although z/OS has built-in support for Parallel Sysplex clustering. Source: Internet
In practice many customers use multiple mainframes linked either by Parallel Sysplex and shared DASD (in IBM's case), or with shared, geographically dispersed storage provided by EMC or Hitachi. Source: Internet