Definitions

Many of the arguments over free hardware design consist of people from software and hardware backgrounds talking at cross-purposes. One cause of this is the simple fact that the word `software' refers both to source code and executables, while the words `hardware' and `hardware design' clearly refer to two different things. Using the word `hardware' as short-hand for both design and physical object is a recipe for confusion. The following terms have all been used in discussions of this topic.

Free hardware design refers to a design which can be freely copied, distributed, modified, and manufactured. It does not imply that the design cannot also be sold, or that any hardware implementation of the design will be free of cost. All the same arguments about the meaning of 'freedom' between supporters of the Free Software Foundation, and the supporters of BSD_style licensing for software unfortunately carry over to hardware designs.
Libre hardware design refers to the same class of design as free hardware design, but tries to make it clear that the word free refers to freedom, not price. The term sounds very clumsy to many English ears, but is of course natural for French speakers (and translations of it are natural to speakers of most languages, which do not use one word in both free beer and free speech).
Open source hardware refers to hardware for which all the design information is made available to the general public. Open source hardware may be based on a free hardware design, or the design on which it is based may be restricted in some way.
Open Hardware is a trademark of the Open Hardware Specification Program. It is a limited form of open source hardware, for which the requirement is that:
Sufficient documentation on the device must be available for a competent systems programmer to write a device driver. The documentation must cover all of the features of the device-driver interface that any user would be expected to employ. This includes input/output and control functions and auxiliary functions such as performance measurement or self-test diagnostics. Details of on-board firmware and the hardware implementation need not be disclosed except when necessary to make it possible to program a driver for the device.
That is, only a limited amount of information about the design need be available; possibly not enough, for example, to attempt a repair.
Free hardware is a term occasionally used as a synonym for open source hardware. It is a tempting term, since 'hardware' appears to be directly parallel to 'software', but suffers from blurring the distinction between design and implementation. The term free hardware is particularly confusing since it implies that the physical hardware, rather than its' design, is somehow free. This is not normally true in the cost sense, and is meaningless (except metaphorically) in a social sense. It seems simpler to avoid this term completely, except in it's cost meaning, as with the free computers given away by various social organizations.