The Budapest Open Access Initiative definition:
By 'open access' ... we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited.
Also, see "Open Access Overview" by OA champion Peter Suber.
There are 4 primary modes of Open Access
- Open Access Publishing: Authors can choose to publish their research articles in a growing number of journals that meet the full definition of Open Access. Articles are free to all interested readers, and the publishers place no financial or copyright barriers between the readers and the article.
- Digital Repositories: Authors can choose to deposit their research articles in digital archives (often called Digital Repositories or Institutional Repositories) which conform to the standards of the Open Archives Initiative (OAI), and enable readers to freely access and fully reuse the article text.
- Effectively Managed Author Rights: As the authors of a research paper, you have ability to ensure that your article can be accessed and used by the widest possible audience.
- Local, National and International Open Access Policies: Institutions that support research, from public and private research funders to higher education institutions, can implement effective policies that that support making Open Access to scholarly research articles the default mode for their researchers.