Open access (OA) is a set of principles and a range of practices through which nominally copyrightable publications are delivered to readers free of access charges or other 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, which regulates post-publication uses of the work. The main focus of the open access movement has been on "peer reviewed research literature", and more specifically on academic journals.
https://en.wikipedia.org/wiki/Open_access