A permissive free software licence is a free software licence for a copyrighted work that offers many of the same freedoms as releasing a work to the public domain. In contrast, copyleft licences like the GNU General Public License require copies and derivatives of the source code to be made available on terms no more restrictive than those of the original licence.
Well-known examples of permissive licences include the MIT Licence and the BSD licences.
Comparison to public domain
Computer Associates Int'l v. Altai
used the term "public domain" to refer to works that have become widely shared and distributed under permission, rather than work that was deliberately put into the public domain. However, such licences are not actually equivalent to releasing a work into the public domain
Some permissive free software licences contain clauses that require advertising materials to credit the copyright holder. Licences with an advertising clause include the 4-clause BSD license
, the PHP License
, and the OpenSSL License
. These licences, although they are permissive free software licences, are incompatible with the widely used GNU General Public License
Examples of permissive free software licences without advertising clauses are the MIT License, the 3-clause BSD license, the Zlib License, and all versions of the Apache License except 1.0.