Open access (OA) is freely available, digital, and typically scholarly content. Entire journals or digital platforms may be OA, but even many traditionally published journals now allow some content to be freely archived in repositories.
OA access comes in many varieties. Publishers may distinguish between:
Green OA Self-archiving of pre-publication or published works to an institutional or disciplinary archive. The publisher stipulates whether pre-publication, uncorrected proof, or final publication may be uploaded.
Gold OA Publishing in an entirely OA journal.
Gratis OA is available free of charge to the reader though copyright and licensing restrictions may still apply.
Libre OA is also free to the reader, but also free of most copyright and licensing restrictions.