This is an attempt to create a definition what is free (as in "freedom") data format, similar to some existing definitions on what is free software. (Actually, the definition of Free Software Foundation on what is free software was its inspiration and model.) It is by no means final or perfect; any criticism is warmly welcome.
A data format is free, if it matches all of the following conditions:
1. It can be freely used by anyone, for any purposes.
A data format is actually useful for only one purpose - storing data according to it. Here, "for any purposes" means that no legal limitations may be imposed on what data may be stored in this format.
2. It can be freely studied.
No legal limitations may be imposed on studying this data format and/or its description/blueprint. (The studying of some specific data, stored in it, may be restricted, if the studying of these data is restricted, regardless of the data format.)
4. It can be freely adapted, modified and/or extended. (Valid both for the format itself, and its complete blueprint/description.)
No legal limitations may be imposed on these activities, based on the fact that it is this format that is used. (The adaptation etc. of some specific data may be restricted, but not due to the format they are stored in.)
4. It can be freely distributed, in original or changed form. (Valid both for the format itself, and its complete blueprint/description.)
No legal limitations may be imposed on the distribution of this data format, and/or its derivatives. (The distribution of some specific data itself may be restricted for some reason, regardless of its format. However, the distribution of the format structure by itself is free.)
The same is valid
about the distribution of the complete data format blueprint, or
description. Being an explanation of the essence of the data format,
its freedom is inseparable from the freedom of the data format
Humbly suggested by Grigor Gatchev