"Open Access is the free, immediate, online availability of research articles combined with the rights to use these articles fully in the digital environment. Open Access is the needed modern update for the communication of research that fully utilizes the Internet for what it was originally built to do—accelerate research" - SPARC
The open access movement is transforming the traditional model of scholarly publishing and challenging established norms for the access, sharing, and re-purposing of knowledge. Open access guarantees research is available online at no cost and permits the transformation of ideas to improve society.
An explanation from the Scholarly Publishing and Academic Resources Coalition (SPARC) on how open access is "critical to accelerating the way we discover knowledge and unlocking our potential to solve big problems and make new discoveries."
A Science Insider article by Jeffrey Brainard providing an overview of a 2024 study reporting that open-access papers have a greater reach than paywalled ones in two key ways: They attract more total citations, and those citations come from scholars in a wider range of locations, institutions, and fields of research.
In order to help reduce the gap between industrialized countries and those in the emerging economy, UNESCO has decided to adopt an Open Access Policy for its publications.

Gold Open Access
Gold OA is open access provided by journals. These journals make articles available for free to readers when they are published. Authors may be asked to pay an article processing charge to have their article published with gold open access.
Green Open Access
Green OA is open access driven by the author. Some journal policies allow authors to archive pre-print or post-print versions of their articles. Sometimes temporary embargoes are required. You can check a journal's open access and archiving policies on the Open Policy Finder website.
Hybrid Open Access
Hybrid OA is a type of Gold OA where a subscription journal gives authors the option to make individual articles freely accessible to readers, for an additional fee. Other articles in the journal remain accessible only through subscription. This may mean that the author pays for publication and the subscribers to the journal continue to pay the same price for access to articles from the journal, resulting in publishers getting paid twice for the articles published this way.
Diamond Open Access
Diamond OA is a publication model in which journal platforms do not charge either the authors or the readers fees to publish or read articles. These journals are community-driven, academic-led, and academic-owned publishing initiatives that are equitable.
This 8 minute video by Jorge Cham (PhD Comics) featuring Nick Shockey and Jonathan Eisen provides an excellent overview of open access.