According to Peter Suber open access is academic literature which is “digital, online, free of charge, and free of most copyright and licensing restrictions“. Open access delivered by journals is called “gold” open access and open access delivered by repositories is called “green” open access. Most academic literature is not open access. And in recent years there has been a growing open access movement to remove paywalls, which are put up by journal publishers.
It remains difficult to be certain of the exact amount of academic research sitting behind paywalls, but the toll-access model still dominates. Although many funders, including various governments, are attempting to bring about an open access revolution, we will remain in a period of transition for many years to come. During this time, a mix of open access and traditional subscription models will operate side-by-side.