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Open Access refers to resources that are freely available for viewing and/or use. Open Access is not the same as Public Domain, and most Open Access creators do retain their copyrights.
Open Access is part of a continuum ranging from completely closed, subscription/purchase only access to completely open, no barrier publishing. Open Access is not related to the quality of materials or the peer-review/non-peer-review status of the publications.
Open Access differs from Open Source or Open Licensed in that the primary use of Open Access materials will be viewing or reading, while Open Source materials are available for modification of the underlying software or text.