[sillymath]

> What does volume mean in higher dimensions?

I think you need a scalar product to define volume. But fuzzy thinking an analogy: Think friendship, define three concepts related to friendship and that they are orthogonal, now establish some numeric scale to measure each concept, then you have just defined a volume to measure the degree of friendship. Unfortunately there is no canonical way to establish those orthogonal features, but they could be obtained applying a linear model to big data sets of measures of friendship.