Open means anyone can freely access, use, modify, and share for any purpose. Open information is, at most, subject to protocols that preserve provenance and openness. Source: Open Knowledge Foundation

Knowledge is open if anyone is free to access, use, modify, and share it — subject, at most, to measures that preserve provenance and openness.

This essential meaning matches that of “open” with respect to software as in the Open Source Definition and is synonymous with “free” or “libre” as in the Definition of Free Cultural Works. The Open Definition was initially derived from the Open Source Definition, which in turn was derived from the Debian Free Software Guidelines. Source: Open Definition

Open data is data that can be freely used, re-used and redistributed by anyone – subject only, at most, to the requirement to attribute and share-alike. 

Share-Alike license allows re-distribution and re-use of a licensed work on the conditions that the creator is appropriately credited and that any derivative work is made available under “the same, similar or a compatible license”. Source: Open Data Handbook

Open government strategies and initiatives are based on the principles of transparency, integrity, accountability and stakeholder participation.:  Source: Organisation for Economic Co-operation and Development (OECD)

Five characteristics serve as the basic conditions for openness in most contexts:

• Transparency
• Inclusivity
• Adaptability
• Collaboration
• Community

Source: The Open Organization