A Business Case is a story that says "this is why we should spend money if on X".

But if you need to talk to a Manager, then you should probably describe a business case as "an internally self-consistent and self-evident set of arguments, propositions and accepted facts, grounded in an articulation of the nature of a business problem, delivering justification for the expenditure inherent in a proposed initiative".  Otherwise they won't understand you.

A ConceptOfOperations is a related idea.