Definition of Green OA from the UC Libraries' Pathways report:
"Green open access is repository-based open access. Green OA models are agnostic about publisher open access behaviors, relying instead on institutions and authors to take steps to make otherwise toll-access works freely available in online repositories that may be (and often are) managed by institutions. In essence, successful green open access requires: the right to share a given scholarly output, a copy of it, the motivation to share it, and a location for sharing it (i.e., a repository)."
Traditionally, the copyright to scholarly articles was transferred to the publisher, which prevented the author from placing their work in a repository ("self-archiving"). Today, many journals allow self-archiving by default, often with limitations on when, where, and what version of the article you can self-archive. When a journal's default agreement does not permit self-archiving, many authors negotiate to retain that right.
Publishing agreements often distinguish between three different versions of an article when describing what self-archiving is acceptable:
Academic social networks, such as Academia.edu and ResearchGate, differ from open access repositories. They are typically operated on a for-profit basis and do not have the same preservation commitments as repositories hosted by academic institutions. The following articles provide more information about these distinctions.