Open Access is the free, immediate, online availability of research articles, coupled with the rights to use these articles fully in the digital environment.
(SPARC, the Scholarly Publishing and Academic Resources Coalition)
Open access (OA) literature is a method of sharing scholarship that is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes OA possible is the internet and the consent of the author or copyright-holder. OA is entirely compatible with peer review, and all the major OA initiatives for scientific and scholarly literature insist on its importance.
(A Very Brief Introduction to Open Access by Peter Suber, a leading OA advocate)
A short video by Nick Shockey and Jonathan Eisen at PhD Comics.