Systems engineering (also known as systems design engineering or SE) is
[t]he overarching process that a program team applies to transition from a stated capability to an operationally effective and suitable system. SE encompasses the application of SE processes across the acquisition life cycle (adapted to each and every phase) and is intended to be the integrating mechanism for balanced solutions addressing capability needs, design considerations and constraints, as well as limitations imposed by technology, budget, and schedule. The SE processes are applied early in concept definition, and then continuously throughout the total life cycle.
an interdisciplinary approach aimed at enabling the realization of successful systems. It focuses on defining client needs and required functionality to address those needs early in planning, and then carries out design and operation while considering the complete problem from both business and technical perspectives.
[an] [i]nterdisciplinary approach governing the total technical and managerial effort required to transform a set of stakeholder needs, expectations, and constraints into a solution and to support that solution throughout its life.