A system's central concepts working together as a smooth, cohesive, emergent whole.

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When high level concepts are combined into a greater whole, they form a certain invisible brainstuff that Fred Brooks calls "Conceptual Integrity". In his classic 1975 book "The Mythical Man Month" he says,

Note Fred's italics on the word "the".

Conceptual Integrity cannot be completely physically captured, even in all of a system's code and artifacts. It's essence remains in the minds of its creators or their direct decendents, and they must be present at various decision points in a system's lifecycle. This is why Microsoft organizes projects into Program Manager (responsible for Conceptual Integrity of the software) and Project Manager (responsible for successful completion of the project). If the two are combined the first invariably gets short changed.

For example the best films are made by strong directors. They are the Program Manager and the producer is the Project Manager.

Instill and preserve Conceptual Integrity at all costs whenever something large really matters.