A StrongConcept is a Concept that has evolved to the point where it never changes as is it only ever enhanced.  At a lower level, its implementation may change but its effect can be depended upon.

A StrongConcept is the generalisation of a particular design pattern and a particular interface.

A particular DesignPattern or a particular Interface is the realisation of some concept.

Correct and useful DesignDocumentation depends on StrongConcept''''''s.

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An example is the some sort of transactional pattern to ensure ACIDity.  Once the concept (patterns + interfaces) are understood and they are not subject to change then other design entities can depend upon that transactional concept.