Open source hardware, what is it?

Open source hardware is a term we've used here on MAKE & CRAFT for describing some of the projects featured as well as some of our electronics kits and it was the subject of a talk we participated in at then SXSW conference, but what is it?

There are a few definitions, some come from "open source software" which is usually considered software's - "source code under a license (or arrangement such as the public domain) that permits users to study, change, and improve the software, and to redistribute it in modified or unmodified form"

Open source hardware, what is it?