The Open Definition, first released by Open Knowledge in 2005, sets out under what conditions data and content is open. Both legal and technical compatibility is vital, and the Open Definition ensures that openly-licensed data can be combined successfully, avoiding a proliferation of licences and terms of use for open data leading to complexity and incompatibility.

You are free: To share: To copy, distribute and use the database, To create: To produce works from the database, To adapt: To modify, transform and build upon the databases, as long as you: Blank: This section is intentionally left blank. The PDDL imposes no restrictions on your use of the PDDL licensed database. If you use the PDDL for any changes you make, the work stays free and open for all. CC0 is compliant with the Science Commons protocol for open data, as is the Open Data Commons PDDL.

You are free: To share: To copy, distribute and use the database, To create: To produce works from the database, To adapt: To modify, transform and build upon the databases, as long as you: Attribute.

An open licence specifically directed at the use of databases. You are free: To share: To copy, distribute and use the database, To create: To produce works from the database, To adapt: To modify, transform and build upon the databases, as long as you: Attribute (You must attribute any public use of the database, or works produced from the database, in the manner specified in the ODbL.

Open Data Commons, an Open Knowledge Foundation initiative, is the home of a set of legal tools and licenses to help you publish, provide and use open data.

Data is open if it can be freely accessed, used, modified and shared by anyone for any purpose - subject only, at most, to requirements to provide attribution and/or share-alike.

The principle that access to the published papers and other results of research, especially publicly-funded research, should be freely available to all. This contrasts with the traditional model where research is published in journals which charge subscription fees to readers. Besides benefits similar to the benefits of open data, proponents suggest that it is immoral to withhold potentially life-saving and valuable research from some readers who may be able to use or build on it.

A formal model that allows knowledge to be represented for a specific domain. An ontology describes the types of things that exist (classes), the relationships between them (properties) and the logical ways those classes and properties can be used together (axioms).

The Open Government Partnership, a partnership of national governments launched in 2011 with the aim of promoting open government in the member countries and collaborating on multi-lateral agreements and best practices. It covers also open data.

Open Database Licence, an attempt to create an open licence for data which covers the ‘database rights’ as well as copyright itself. It does this by imposing contractual obligations on the data re-user.