Budapest Open Access Initiative
The BOAI first defined open access in 2002: "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 BOAI includes two primary strategy recommendations for achieving OA: self-archiving by authors (in OA repositories) and supporting OA journals.
The Scholarly Publishing and Academic Resources Coalition (SPARC) defines Open Access (OA) as "the free, immediate, online availability of research articles coupled with the rights to use these articles fully in the digital environment. Open Access ensures that anyone can access and use these results—to turn ideas into industries and breakthroughs into better lives." USD is a proud member of SPARC.
This guidebook by the Authors Alliance provides an accessible, comprehensive overview of OA, and here's a solid Open Access Overview by Peter Suber.
Finally, for a quick video introduction to OA and why it's so important, watch this: