The term open access refers to the unrestricted access and reuse of peer-reviewed scientific journal articles. Reading is free - without paywalls or subscription fees; reuse is legitimized and usually governed by a Creative Commons Attribution license. Under such a license, authors retain the copyright and give users perpetual rights to read, copy, distribute and further use an article, as long as the work is properly cited. The publisher has a non-exclusive license to publish the article. There can be some restrictions on further use such as prohibiting commercial use. In addition, authors are allowed to deposit their articles in any repository (e.g. university), and reuse their own figures and text without needing explicit permission from a publisher. Shortly after publishing, the articles are usually archived and indexed in public repositories, such as PubMed Central. Open access articles are fully accessible by search engines and often available for text mining, which further enhances the use of the results and their visibility.
The different types of open access publishing are often referred to as platinum/diamond, gold and green: