• Systems engineering is a methodical approach to the specification, design, creation, and operation of a function.
• System engineering is a robust approach to the design, creation, and operation of systems. In simple terms, the approach consists of identification and quantification of system goals, creation of alternative system design concepts, performance of design trades, selection and implementation of the best design, verification that the design is properly built and integrated, and post-implementation assessment of how well the system meets (or met) the goals. — NASA Systems Engineering Handbook, 1995.

• "The Art and Science of creating effective systems, using whole system, whole life principles" OR "The Art and Science of creating optimal solution systems to complex issues and problems — Derek Hitchins, Prof. of Systems Engineering, former president of INCOSE (UK), 2007.

Definitions:  Systems security engineering is a specialty engineering field strongly related to systems engineering. It applies scientific, engineering, and information assurance principles to deliver trustworthy systems that satisfy stakeholder requirements within their established risk tolerance(NIST).