Open access means the open availability to the public, without charge, typically of scholarly publications--usually books and journal articles. In addition, open access signals the relatively unrestricted reuse of the scholarly work in, for example, new scholarly output.
Peter Suber provides a thorough definition of OA. Here are a few of the key points:
1. Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions. There is some flexibility about which permission barriers to remove. For example, some OA providers permit commercial re-use and some do not.
2. The publishing costs (still extant) are not paid by readers and hence do not function as access barriers.
3. The legal basis of OA is the consent of the copyright holder (for newer literature) or the expiration of copyright (for older literature). Because OA uses copyright-holder consent or the expiration of copyright, it does not require the reform, abolition, or infringement of copyright law.
4. The focus of OA is royalty-free literature. It is the journal articles and sometimes books and chapters that scholars contribute to the scholarly body of literature without seeking remuneration. Scholars write articles and chapters and sometimes entire books because advancing knowledge in their fields advances their careers. They write for impact, not for money. OA applies to any scholarly literature to which authors forego royalties.
5. OA is compatible with peer review, and all the major OA initiatives for scientific and scholarly literature insist on its importance.
6. Gratis means OA that is free, but with reuse restrictions. Libre means OA that is free, and without most reuse restrictions.
7. There are two primary vehicles for delivering OA publications: OA journals ("gold OA") and OA repositories ("green OA").
8. OA is really not a kind of content. Every kind of digital content can be OA, from texts and data to software, audio, video, and multi-media. The OA movement focuses on peer-reviewed research articles and their preprints. While most of these are just text, a growing number integrate text with images, data, and executable code. OA can also apply to non-scholarly content, like music, movies, and novels, even if these are not the focus of most OA activists.
According to the Right to Research Coalition,
Open access is made possible in a number of ways, including:
Open-access journals that make their articles freely available at the time of publication
Open digital repositories, where articles are deposited and can be indexed and accessed by anyone with an Internet connection
Effectively managed copyright – authors are retaining the rights they need to ensure their articles can be accessed and used by the widest possible audience, and be published in the most prestigious venue
Local, national and international policies that support Open Access to scholarly research results