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."
In other words, Open Access literature is both free to read online and free from most copyright restrictions. However, 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).
Increased Visibility and Impact: Open Access boosts global visibility, reaching diverse audiences, and often leads to increased citations, enhancing the overall impact of your research. Our own findings suggest that, on average, OA publications receive 2.4 times the citations of those received by non-OA publications (data: VU publications in Scopus from 2020-2022).
Rapid Dissemination: Open Access facilitates the rapid dissemination of research findings. This timely sharing of knowledge allows for quicker advancements in the respective field and accelerates the research cycle.
Collaboration and Interdisciplinary Research: Open Access fosters collaboration by removing barriers, enabling scholars from diverse fields to easily access and build upon each other's work.
Public Engagement: Open Access promotes knowledge dissemination beyond academia, allowing the general public to access and benefit from scholarly research, aligning with principles of community engagement.
Compliance with Funding Mandates: Open Access ensures compliance with funding agency and institutional mandates, securing continued support for your research.