I think of acutance as the derivative (slope) of negative density across a sharp boundary that should reach theoretically DMax on one side and DMin on the other side of the boundary. So you put a sharp edge (knife) on the film, expose, develop, and see what you get. If everything were perfect, you should get something like a step function, but in reality you get something with a smooth slope because the materials don't have perfect resolution, there is some smear effect from developing, and there's probably also some light leakage along the edge too.
So in other words you just want to know how much the film and the development smears out what should be a very abrupt change of tone from one extreme to the other.
So my way of thinking seems to agree with Ian's very closely.