Abstract Noun:
* When it refers to a plan or system of ideas, it's abstract. For example: "His scheme to win the lottery was elaborate." Here, "scheme" represents an intangible plan.
Concrete Noun:
* When it refers to a specific plan or design, it can be concrete. For example: "The scheme of the building included a rooftop garden." Here, "scheme" refers to a tangible, specific plan for the building's design.
In summary: "Scheme" can be either abstract or concrete depending on the context and its meaning in the sentence.