The terms hardwood and softwood are often used, but I struggle to find a reliable definition.
Is the wood's density sufficient as the distinguishing feature?
Is it the only one?
As oil is known to harden a wooden surface, is it possible to create a (in terms of density) hardwood-like surface on a piece of softwood? Is it even possible to get softwood that's harder than (relatively soft) hardwood this way?
When I asked for hardwood and softwood, I had the German terms “Hartholz” and “Weichholz” in mind. It turns out they do not translate one-to-one: In English, according to Wikipedia and ON5MF's answer, the distinction is made by the type of tree from which the wood is harvested. That corresponds to the German terms “Laubholz” (wood from dicot angiosperm trees, = hardwood) and “Nadelholz” (wood from gymnosperm trees, = softwood). The differentiation in “Hartholz” and “Weichholz” differs from that in a way that it takes the wood's “hardness” as the distinguishing feature, and I wondered how exactly this is measured and defined. So, additional question: Is there a similar differentiation in English?
tl;dr: How do you measure a wood's hardness?