From the Open Data Manual:
Open data needs to be ‘technically’ open as well as legally open. Specifically the data needs be:
- Available — at no more than a reasonable cost of reproduction, preferably for free download on the Internet. Summary: publish your information on the Internet wherever possible.
- In bulk. The data should be available as a whole (a web API or service may also be very useful but is not a substitute for bulk access)
- In an open, machine-readable format. Machine-readability is important because it facilitates reuse, for example, tables of figures in a PDF can be read easily by humans but are very hard for a computer to use which greatly limits the ability to reuse that data.
Sir Tim Berners-Lee’s linked open data star scheme provides an unambiguous way to categorize open data. And while I’m at open data there’s also the Open Data Protocol which is meant to enable the creation of HTTP-based data services.