Open access (OA) refers to online research outputs that are free of all restrictions on access (e.g. access tolls) and free of many restrictions on use (e.g. certain copyright and license restrictions).
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, and monographs.
Two degrees of open access can be distinguished: gratis open access, which is online access free of charge, and libre open access, which is online access free of charge plus various additional usage rights.
These additional usage rights are often granted through the use of various specific Creative Commons licenses.
Libre open access is equivalent to the definition of open access in the Budapest Open Access Initiative, the Bethesda Statement on Open Access Publishing and the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities.