The Budapest Open Access Initiative defines Open Access:
""Open access" to [research] literature, ... means its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself."
Some subdivide this definition into two flavors of open access: gratis and libre. This distinction allows freely available content with non-permissive licensing to qualify as Open Access.
Gratis open access literature is free to access, download, read, and save, but does not come with a permissible license allowing its reuse.
Libre open access literature meets the definition on gratis OA and is free use. These works are often released under a Creative Commons License. The Creative Commons Attribution and Attribution/Share Alike licenses meet the criteria to allow free use.
Green open access means archiving an open access version of a work in a subject or institutional repository. Most publishers allow for self archiving of some version of an article. If the publishing agreement does not allow for self-archiving, an author can negotiate the retention of this right by using an addendum to the publishing contract.
Gold open access literature has been published in anopen access journal or released as open access in a toll access journal (aka a hybrid journal). This method may require an article processing charge (APC).