We define systems in the Preface: "In this book we talk generally about systems, which is a conceptual way of thinking about the groups of components that cooperate to perform some function. In our context of systems engineering, these components typically include pieces of software running on the processors of various computers. They may also include the hardware itself, as well as the processes by which people design, implement, and maintain the systems. Reasoning about the behavior of systems can be difficult, since they’re prone to complex emergent behaviors."

(Book author here)