Open Access (OA) is a publishing model that makes digital literature freely available online rather than behind a paywall. There are levels of open access which can indicate the following:
It's important to note that the OA movement is changing and evolving all the time! Levels are added and redefined as time goes on and the publishing landscape changes!
Jamie-farquharson, CC BY 4.0., via Wikimedia Commons
Open access policies can vary widely so it is important to familiarize yourself with each journal's specific open access policy if you are interested in publishing with them. You can go to the journal's website to find open access policy but there are other sites that can help you find open access policies.