Open access (OA) is a set of principles and a range of practices through which research outputs are distributed online, free of cost or other access barriers. With open access strictly defined (according to the 2001 definition), or libre open access, 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 conventional (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. 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,[3] book chapters,[1] and monographs. Green OA is gratis for the author. Some publishers (less than 5 % and decreasing as of 2014) may charge a fee for an additional service[10] such as a free license on the publisher-authored copyrightable portions of the printed version of an article.

If the author posts the near-final version of their work after peer review by a journal, the archived version is called a "postprint". This can be the accepted manuscript as returned by the journal to the author after successful peer review.