Open access (OA) is a set of principles and a range of practices through which research outputs are distributed online, free of access charges or other barriers. Under some models of open access publishing, barriers to copying or reuse are also reduced or removed by applying an open license for copyright.

The main focus of the open access movement is "peer reviewed research literature". Historically, this has centered mainly on print-based academic journals. Whereas non-open access journals cover publishing costs through access tolls such as subscriptions, site licenses or pay-per-view charges, open-access journals are characterised by funding models which do not require the reader to pay to read the journal's contents, relying instead on author fees or on public funding, subsidies and sponsorships. Open access can be applied to all forms of published research output, including peer-reviewed and non peer-reviewed academic journal articles, conference papers, theses, book chapters, monographs, research reports and images.

Since the revenue of most open access journals is earned from publication fees charged to the authors, OA publishers are motivated to increase their profits by accepting low-quality papers and by not performing thorough peer review. On the other hand, the prices for OA publications in the most prestigious journals have exceeded 5,000 US$ per article, making such publishing model unaffordable to a large number of researchers. This increase in publishing cost has been called the "Open-Access Sequel to [the] Serials Crisis".