A set of things working together as parts of a mechanism or an interconnecting network. A System will include Application Components and Technology Components, and sometimes, people.
Definitions
You can specify these related definitions for a System:
•Architecture Building Blocks it maps to.
•Solution Building Blocks it maps to.
•Line of Business that owns it (Organization Unit).
•Technology Components it is comprised of.
•Processes that implement the system.
Diagrams
You can visualize Systems on these TOGAF diagrams: