Public domain is a legal term that describes a work or product that is not protected by copyright. The copyright protection an item in the public domain may have 1) expired, 2) been released by the author, or 3) never existed in the first place. Public domain items are publicly available and can be freely accessed and redistributed.
Many different items can be labeled as "public domain." For example, books, speeches, poems, artwork, songs, and videos can all be made freely available to the public. In the computing world, "public domain" is often used to refer to software programs that are offered to the public without copyright restrictions.
Public domain software is similar to open source software, in which the source code of a program is made publicly available. However, open source software, while freely distributed, still retains the original developer's copyright. This means the developer can change the redistribution policy at any time. Public domain software is also similar to freeware, which refers to software offered at no charge. However, like open source software, freeware programs are still protected by copyright. Therefore, users may not redistribute the software unless they receive permission from the original developer.
Since there are many similarities between freeware, open source, and public domain software, the terms are often used interchangeably. However, there are important legal differences between the licenses, so it is important for developers to choose the correct license when releasing software programs. Public domain software, which offers the least legal protection, is most often published by individuals or educational institutions, rather than companies. When software is offered as public domain, it is often labeled "PD" or may include a Public Domain Mark (PDM).