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Open access (OA) refers to unrestricted online access to articles published in scholarly publications. Types of open access publications available online include:
books and book chapters
"By 'open access' ... we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search or link to the full text of these articles, crawl them for indexing, pass them as data to software or use them for any other lawful purpose…" The Budapest Open Access Initiative