"A piece of data is open if anyone is free to use, reuse, and redistribute it ---
subject only, at most, to the requirement to attribute and/or share-alike." (source)
The principle of Open Data is based on the idea that certain data should be freely availble for anyone to publish or reuse as they wish. It is similar to other Open movements such as Open Source, Open Software, Open Access, and more (source).
In 2007, thirty open government advocates gathered in Sebastopol, California and wrote a set of eight principles of open government data. A brief annotation of these principles can be found below:
Government data shall be considered open if it is made public in a way that complies with these principles:
All public data is made available that is not subject to valid privacy, security, or privilege limitations. Non-electronic resources are not subject to these principles, but it is encouraged that these resources be made available electronically as feasible.
Data is collected at the source with the highest possible level of granularity. If an entity chooses to transform data by aggregation or transcoding, it still has an obligation to make the full-resolution data available for others and to preserve the data for posterity.
Data is made available as quickly as necessary to preserve the value of the data.
Data is available to the widest range of users for the widest range of purposes.
5. Machine Processable
Data is reasonably structured to allow automated processing.
Data is available to anyone, with no requirement of registration.
Data is available in a format over which no entity has exclusive control.
Data is not subject to any copyright, patent, trademark, or trade secret regulatoin. Reasonable security, privacy, and privelege restrictions may be allowed.
Compliance with these principles must be reviewable.
You can find more information on these principles and Open Data in general here: http://opengovdata.org/