The idea of open and open-source hardware has been growing in popularity and practice, but it's not always clear what is meant by the terms. Make laid out something of a choose-your-own-definition, but it's difficult to use a term which can simultaneously refer to a multitude of possibly incompatible meanings.
To me, the definition of "open-source hardware" is straightforward, and analogous to that of open-source or free software. It is, simply, the provision of the digital artifacts necessary to reproduce, understand, and modify a piece of hardware. As in software, the open-source design may depend on closed or proprietary components (e.g. an operating system or a integrated circuit) so long as those may be incorporated into the copy or derivative of the original open-source work. I would exclude, however, those products for which the information provided allows only for the use or extension, but not the reproduction or modification of the original. It's not enough to tell people how your hardware works, you have to enable them to build it for themselves.
Finally, a few notes from my experiences on Arduino, an open-source hardware and software platform for electronics prototyping. Open-source does not entail democracy: you can make your own, so you don't (necessarily) get a vote on how I make mine. In open-source hardware, forking can be a valuable method for making more products available to the community, rather than a last resort in the face of irresolvable differences. Additionally – and this is crucial when many entities sell competing versions of compatible products – open-source does not imply the right to another group's identity. As with open-source software, you can use a project's work as the basis for another, but you can't claim that it's part of the original.
I hope to write more about topics related to open-source hardware, but felt it was important to start by defining the term. Check back for more.