Open access (OA) is a set of principles and a range of practices through which research outputs are distributed online, free of access charges or other barriers. Publishing a scientific and scholarly document under Open Access conditions gives anyone permission to read, download, save, link to, print,  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 should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited
Open access is a publishing model for scholarly communication that makes research information available to readers at no cost, as opposed to the traditional subscription model in which readers have access to scholarly information by paying a subscription (usually via libraries). 

Source: Budapest Open Access Initiative (2002). Budapest Open Access Initiative Declaration.
Online: https://www.budapestopenaccessinitiative.org/read.