Software System Integration (SSI) is an IT or engineering process or phase concerned with joining different subsystems or components as one large system. It ensures that each integrated subsystem functions as required.
SSI refers to the practice of combining individually tested software components into an integrated whole. Software is integrated when components are combined into subsystems or when subsystems are combined into products. Components may be integrated after all are implemented and tested as in a waterfall model or a "big bang" approach.