Unlicense

The Unlicense is a public domain equivalent license with a focus on an anti-copyright message. It was first published on January 1 (Public Domain Day), 2010. The Unlicense offers a public domain waiver text with a fall-back public-domain-like license, inspired by permissive licenses but without an attribution clause. In 2015, GitHub reported that approximately 102,000 of their 5.1 million licensed projects (2% of licensed projects on GitHub.com) use the Unlicense.
History.
In a post published on January 1 (Public Domain Day), 2010, Arto Bendiken outlined his reasons for preferring public domain software, namely: the nuisance of dealing with licensing terms (for instance license incompatibility), the threat inherent in copyright law, and the impracticability of copyright law.
On January 23, 2010, Bendiken followed-up on his initial post. In this post, he explained that the Unlicense is based on the copyright waiver of SQLite with the no-warranty statement from the MIT License. He then walked through the license, commenting on each part.
In a post published in December 2010, Bendiken further clarified what it means to "license" and "unlicense" software.
On January 1, 2011, Bendiken reviewed the progress and adoption of the Unlicense. He admits that it is "difficult to give estimates of current Unlicense adoption" but suggests there are "many hundreds of projects using the Unlicense".
In January 2012, when discussed on OSI's license-review mailing list, the Unlicense was brushed off as a crayon license. A request for legacy approval was filed in March 2020, which led to a formal approval in June 2020.
License terms.
The license terms of the Unlicense is as follows:
Reception.
The Free Software Foundation states that "Both public domain works and the lax license provided by the Unlicense are compatible with the GNU GPL." However, for dedicating software to the public domain it recommends CC0 over the Unlicense, stating that CC0 "is more thorough and mature than the Unlicense".
The Fedora Project recommends CC0 over the Unlicense because the former is "a more comprehensive legal text".
Google does not allow contributions to projects under public domain equivalent licenses like the Unlicense (and CC0), while allowing contributions to 0BSD licensed and US government PD projects.
In December 2010, Mike Linksvayer, the vice president of Creative Commons at the time, wrote in an identi.ca conversation "I like the movement" in speaking of the Unlicense effort.
The Unlicense has been criticized, for instance by the OSI, for being possibly inconsistent and non-standard, and for making it difficult for some projects to accept Unlicensed code as third-party contributions; leaving too much room for interpretation; and possibly being incoherent in some legal systems.
Notable projects that use the Unlicense include youtube-dl, Second Reality, and the Gloom source code.