Joined: 23 Nov 2006 Posts: 19270 Location: Inside the Matrix
A sysplex (usually called a parallel sysplex) is a collection of IBM mainframe computers that work together as a single entity. Last i heard, 32 computers may be configured into a single sysplex. Each sysplex is typically broken into Logical Partitions (LPARs).
A site is a less formal term that usually refers to the place (data center) where the "computer room" is. Another version of site is the id used by the data center and the software vendors of the products used at that site to identify which products are licensed and at what support level(s).