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 litterature, 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 publishing model is also used for books. Generally, the digital version of the book is open access, sometimes after an embargo period, but the printed version still has to be paid for. A study conducted by the Swiss National Science Foundation between 2014 and 2017 found that making the digital version open access does not have a negative impact on printed book sales, and increases the visibility and trackability of the publication.
The Open Access movement was born as a consequence of the serials crisis, and the development of the World Wide Web, which made dissemination of academic publications technically easier. Physicists and computer scientists were forerunners, with the creation of arXiv, a preprint repository, in 1991. In the 2000's, the OA movement has been formalised through 3 major declarations, and the number of OA publications has increased noticeably.