From Peter Suber, Director of the Harvard Open Access Project, "Open access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions".
From the Scholarly Publishing and Academic Resources Coalition (SPARC) Open Access is 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.