Systems Engineering is defined by INCOSE as: "..an interdisciplinary approach and means to enable the realization of successful systems. It focuses on defining customer needs and required functionality early in the development cycle, documenting requirements, then proceeding with design synthesis and system validation while considering the complete problem: Operations, cost and schedule, performance, training and support, test, manufacturing and disposal.
Systems Engineering integrates all the disciplines and specialty groups into a team effort forming a structured development process that proceeds from concept to production to operation. Systems Engineering considers both the business and the technical needs of all customers with the goal of providing a quality product that meets the user needs."
Us here at the Swiss Society of Systems Engineering have a simpler way of expressing Systems Engineering, "It's doing Engineering more efficiently and effectively".