Systems Integration is the application of skills and knowledge of connecting components of an architecture such that it performs its intended function.
Integration is broad-ranging and can refer to the connections of wires, to operating software on a mainframe, or integrating data over a network.
Some technologies like an Enterprise Service Bus or SOA are built for standardising and simplifying integration. However, new technologies are frequently displacing the older methods of getting things done.
Integration Architecture speaks of a framework for integrating components. Such a framework is a toolbox of capabilities for testing, load testing, etc...