INCOSE : International Council on Systems Engineering
Systems Engineering is 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.
Alternate Definitions:

Siemens - Model Based Systems Engineering helps companies develop the complex products their customers demand. Model Based Systems Engineering focuses on creating and exploiting domain models as the primary means of information exchange between engineers.

MBE Maturity Index - A method of system engineering that is based on models.

DODI 5000.97 - The formalized application of modeling to support system requirements, design, analysis, verification, and validation activities beginning in the conceptual design phase and continuing throughout development and later system life-cycle phases.