There are many ways to define open access, but here is the definition given by the Budapest Open Access Initiative:
"By "open access" to this literature, we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited."
The Open Access movement was born out of the serials crisis, and the development of the World Wide Web, which made it technically easier to disseminate academic publications. Physicists and computer scientists led the way with the creation of arXiv, a preprint repository, in 1991. In the 2000s, the OA movement was formalised by 3 major declarations, and the number of OA publications has increased significantly.