We published a number of references in order to arrive at a more precise definition of open knowledge and its various aspects, see the entry here, inspired by the work of the Open Knowledge Foundation.
Here is a valuable refining by ben vershbow in the Future of the Book blog, on the occasion of the MIT conference on The Economics of Open Content:
“I heard that there are two kinds of “open.” Open as in open access — to knowledge, archives, medical information etc. (like Public Library of Science or Project Gutenberg). And open as in open process — work that is out in the open, open to input, even open-ended (like Linux, Wikipedia or our experiment with MItch Stephens, Without Gods).
I heard that “content” is actually a demeaning term, treating works of authorship as filler for slots — a commodity as opposed to a public good.
I heard that open content is not necessarily the same as free content. Both can be part of a business model, but the defining difference is control — open content is often still controlled content.
I heard that if you build the open-access resources and demonstrate their value, the money will come later.
I heard that content should be given away for free and that the money is to be made talking about the content.
I heard that reputation and an audience are the most valuable currency anyway.”