Open Access is the unencumbered online access to scholarly research that is free from most copyright and licensing restrictions. This means free access for anyone to read, download, copy, distribute, print, and display.
Read the Open Access Overview by Peter Suber, Director of the Harvard Open Access Project, for an introduction to open access with a focus on publishing in peer-reviewed research articles. View a video of Peter Suber answering the question What is Open Access.
HowOpenIsIt? is a guide from PLoS, SPARC and OASPA, designed to clarify the definitions of open access.
gold OA - open access in journals with peer review
green OA - open access resources in repositories with no peer review
pale green OA - OA journals that also permit preprint archiving (green) by authors
gray OA - not peer reviewed, does not permit repository archiving
gratis OA - removes price barriers alone ("free as in beer")
libre OA - removes price barriers and at least some permissions ("free as in speech")