Open access (OA) is a set of principles and a range of practices through which research outputs are distributed online, free of cost or other access barriers. Through licensing via an open license (usually a Creative Commons License), freely available outputs can also be legally shared and reused. Hence, open access is more than just free access.
One definition is:
“For scholarly work, Open Access means making all scholarly outputs freely available via the Internet, permitting any user 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 lawful purpose, without financial, legal or technical barriers other than those inseparable from gaining access to the internet itself.”
Since then the technology and legal frameworks associated with open access have been refined, especially the need for associated open licenses.
Use and adapt this graphic: