Full Definition per INCOSE:
A transdisciplinary and integrative approach to enable the successful realization, use, and retirement of engineered systems, using systems principles and concepts, and scientific, technological, and management methods.
The SEBoK offers a Venn Diagram to help clarify the boundary between SE, Systems implementation, and systems management:
Systems Engineering is like strategy, you can do things without it, but those things aren’t likely to result in the outcome you want. There’s an observed negative correlation between what % of a budget is spent on SE and the % that budget overruns by at the completion of the project (SEBoK pg 42).
Systems Engineering has been a discipline since ~1950 when it was established within Bell Labs.