INCOSE defines Systems Engineering as...

Systems Engineering (SE) is a design and management discipline useful in designing and building systems. It was conceived of and introduced by the U.S. Government to counteract the difficulties encountered in the engineering of increasingly large, complex, and inter-disciplinary systems. Systems Engineering has evolved to aid in the design of other types of systems, including small systems. Systems Engineering translates a customer's need, arising from a specific mission or business objective, into a set of requirements and specifications for a system's performance and configuration.