ASME Y14.100 : Engineering Drawing Practices
A composite of equipment, skills, and techniques capable of performing or supporting an operational role or both. A complete system includes all equipment, related facilities, material, software, services, and personnel required for its operation and support to the degree that it can be considered a self- sufficient unit in its intended operational environment.
See Also:
Alternate Definitions:

DAU - An aggregation of system elements and enabling system elements to achieve a given purpose or provide a needed capability.

MBE Maturity Index - An arrangement of elements that exhibits behavior not present in its element.