Open access is free, unrestricted access to high-quality research materials. Information is shared freely and free of charge by or to authors. Authors maintain copyright. Information is not found behind paywalls and typically free of most copyright and licensing restrictions.
“By open access [OA], 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…”.
In other words- royalty-free literature. Research that is available for free to the public in a way that is compatible and in in-compliance with peer-review, copyright, revenue, preservation, prestige, quality, career development, discovery and other factors no different than regular scholarly publication. The primary difference is that the bills are not paid for by readers and hence do not function as access barriers.