Known as the BBB Definitions on open access, these are the key statements informing the OA movement:
Diamond OA refers to publishing an article in a peer-reviewed open access journal that immediately makes the article free at the point of access without charging the authors a fee.
Gold OA refers to publishing an article in a peer-reviewed open access journal that immediately makes the article free at the point of access for a fee.
There is also the Hybrid OA model which involves non-OA journals that offer authors the option of paying an additional fee in order to make their articles open access. SHERPA/RoMEO provides a list of paid OA publishers and the author fees.
Green OA refers to "self archiving" or posting a copy of the article in an online repository. The author may archive either a preprint or postprint of the article depending on permissions. If an author has already published in an non-open access journal, they can usually make their article open access retroactively by depositing it in a repository.
According to Peter Suber, the word "open access" is now used in at least two different ways.
"For some, "OA" literature is digital, online, and free of charge. It removes price barriers but not permission barriers. For others, "OA" literature is digital, online, free of charge, and free of unnecessary copyright and licensing restrictions. It removes both price barriers and permission barriers. It allows reuse rights which exceed fair use.
Gratis OA: is the term to describe OA materials subject to the removal of price barriers alone
Libre OA: describes OA materials subject to the removal of price and at least some permission barriers.
The new terms allow us to speak unambiguously about these two species of free online access."
[Source: SPARC: Gratis and Libre Open Access (2008). Retrieved January 10, 2016 from: http://sparcopen.org/our-work/gratis-and-libre-open-access/]
Spectrum of Open Access