Open access (OA) means unrestricted online access to peer-reviewed scholarly research. Open access is primarily intended for scholarly journal articles, but is also provided for a growing number of theses, book chapters, and scholarly monographs.
Open access comes in two degrees gratis OA, which is free online access, and libre OA, which is free online access plus some additional usage rights. These additional usage rights are often granted through the use of various specific Creative Commons licenses.
The two ways authors can provide OA are (1) by self-archiving their journal articles in an OA repository, also known as 'green' OA, or by (2) by publishing in an open access journal, known as 'gold' OA.