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A Permissive software license, sometimes also called BSD-like or BSD-style license, is a FOSS software licence with minimal requirements about how the software can be redistributed. Examples include the MIT Licence, BSD licences and the Apache licence. As of 2016, the most popular FOSS licence is the permissive MIT licence.
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Definitions
The Open Source Initiative defines a permissive software license as a "non-copyleft license". GitHub's choosealicense website described the MIT permissive license as, "lets people do anything they want with your code as long as they provide attribution back to you and don’t hold you liable." California Western School of Law's newmediarights.com defined them as follows: "The ‘BSD-like’ licenses such as the BSD, MIT, and Apache licenses are extremely permissive, requiring little more than attributing the original portions of the licensed code to the original developers in your own code and/or documentation.".
Copycenter
Copycenter is a term originally used to explain the modified BSD licence, a permissive free software license. The term was presented by Kirk McKusick, a computer scientist famous for his work on BSD, during one of his speeches at BSDCon 1999. It is a word play on copyright, copyleft and copy center.
The way it was characterized politically, you had copyright, which is what the big companies use to lock everything up; you had copyleft, which is free software's way of making sure they can't lock it up; and then Berkeley had what we called ‘copycenter’, which is ‘take it down to the copy center and make as many copies as you want.’
The liberty to 'make as many copies as you want' is in fact also provided by all copyleft licenses. However, unlike both copyleft licenses and copyright law, permissive free software licenses do not control the license terms that a derivative work falls under.
The Copyfree Initiative defines Copyfree as a type of permissive copyright license which falls under their Copyfree Standard Definition.
Copyfree licenses include the Simplified BSD licence, the Open Works license, and others, but not the GNU GPL or other copyleft licenses. The Creative Commons CC0 public domain dedication and the WTFPL license are also considered to be copyfree.
While all copyfree licenses are permissive licenses, not all permissive licenses are copyfree, since they may introduce limitations not allowed under the copyfree definition. Therefore "copyfree" can be seen as subset of "Free and open-source (software)". The Apache Licence 2.0 (as well as previous versions of this license) is an example of a non-copyfree permissive licence. Another notable example is the Creative Commons Attribution 3.0 license, which is also non-compliant with the Copyfree Standard Definition because the license forbids DRM technologies.
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, permissive licenses are not actually equivalent to releasing a work into the public domain.
Permissive licenses often do stipulate some limited requirements, such as that the original authors must be credited (attribution). If a work is truly in the public domain, this is usually not legally required, but a United States copyright registration requires disclosing material that has been previously published, and attribution may still be considered an ethical requirement in academia.
Comparison to copyleft
A major difference between the set of permissive and copyleft free software licenses is that when the software is being redistributed (either modified or unmodified), permissive licenses don't enforce the redistributor to open the modified source code. Copyleft ("sharealike") licenses enforce the publication of the source code under the copyleft license. Some people argue that copyleft licenses see the world as "evil" and therefore enforce "freedoms" ("availability of source code") while permissive licenses see the world as "good", therefore just allowing good actions and hoping for giving back in form of source code. Permissive licenses don't try to guarantee that future generations of the software will remain free and publicly available, in contrast to licenses which have reciprocity / share-alike requirements which try to enforce this.
The FreeBSD project argues on the advantages of permissive licenses for companies and commercial use-cases: they place only "minimal restrictions on future behavior" and aren't "legal time-bombs", unlike copyleft licenses.
Also permissive licenses have often an excellent licence compatibility, in comparison to copyleft licenses who can't be always freely combined and mixed. However, most GPLv2 licensed software allows to upgrade to the terms of later versions of the GPL, achieving by that a somewhat better inter-GPL compatibility. Also, some copyleft licenses have exception clauses that allow combining them with software that is under different licenses or license versions.
License compatibility
In general permissive licenses show a good licence compatibility with most other software licenses in most situations.
Due to their non-restrictiveness most permissive software licenses are even compatible with copyleft licenses, which are incompatible with most other licenses. Copyleft licenses don't allow the addition of additional restrictive clauses which would be often required in a combined work made from copyleft code and other licensed code. Only some older permissive licenses have clauses requiring advertising materials to credit the copyright holder which made them incompatible with copyleft licenses, for instance the 4-clause BSD license, the PHP Licence, and the OpenSSL License. Popular modern permissive licenses, as the MIT Licence, the 3-clause BSD licence, and the Zlib Licence, don't include advertising clauses and are compatible with many copyleft licenses.
Some licenses do not allow derived works to add a restriction that says a redistributor cannot add more restrictions. Examples include the CDDL and MsPL. However such restrictions also make the license incompatible with permissive free software licenses.
Some licenses are permissive but do not qualify as free software licenses as defined by the Free Software Foundation.
Reception and adoption
While always an important part of the FOSS license landscape, in the 2010s years several authors noted a raising popularity of the permissive licenses in contrast to the copyleft license.
As of 2015, the MIT licence, a permissive license, is the most popular license in the FOSS domain before a copyleft one, the second placed GPLv2.