The definition usually given in introductory passages is "produced factor of production".
But then Bob Murphy (and Rothbard too I think) makes the point that if a freak bolt of lightning hewed a spear out of a tree, and people were able to both use it and duplicate it perfectly, it would not be "produced" technically, but it would be economically no different from any other capital good. So then the definition is give: a reproducable factor of production.
But when Rothbard talks about capital goods always wearing out, he says that a produced factor of production that didn't wear out would be for all intents and purposes land.
He also says that the improvable and degradable aspect of "land"-in-the-loose-sense is actually a capital good, and that it is the "site"-aspect of "land" (site land) that is truly land.
So what makes something a capital good? Being produced? Reproducable? Impermanent? Unimprovable? Undegradable?
Anyone here read Capital and It's Structure. I only did one quick read on it. I can't say I disagree with it.