Systems integration is the process of linking together different computing systems and software applications physically or functionally to act as a coordinated whole.

The system integrator brings together discrete systems utilizing a variety of techniques such as computer networking, enterprise application integration, business process management or manual programming.

A system is an aggregation of subsystems cooperating so that the system is able to deliver the over-arching functionality. System integration involves integrating existing (often disparate) subsystems. The subsystems will have interfaces. Integration involves joining the subsystems together by "gluing" their interfaces together. If the interfaces don't directly interlock, the "glue" between them can provide the required mappings. System integration is about determining the required "glue". 

System integration is also about adding value to the system, capabilities that are possible because of interactions between subsystems.

In today's connected world, the role of system integration engineers is becoming more and more important: more and more systems are designed to connect, both within the system under construction and to systems that are already deployed.