Internet Governance Glossary
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It’s the idea that certain data should be freely available to everyone to use and republish as they wish, without restrictions from copyright, patents or other mechanisms of control.
The goals of the open data movement are similar to those of other "Open" movements such as open source, open hardware, open content, and open access. Open data is data that can be freely used, reused and redistributed by anyone - subject only, at most, to the requirement to attribute and sharealike. The key point is that when opening up data, the focus is on non-personal data, that is, data which does not contain information about specific individuals.
Share-alike is a copyright licensing term, originally used by the Creative Commons project, to describe works or licences that require copies or adaptations of the work to be released under the same or similar license as the original. Copyleft licenses are free content or free software licenses with a share-alike condition.