Copyfree is more specific than permissive.
The term "permissive license" is a popular one in the open source software community, and is often used in reference to non-software licenses as well. For most people, it conjures the idea of licenses that "let you do what you want". The way the term is used, however, implies significantly different meaning, applying to many different types of licenses.
The non-specificity of the term "permissive license", the haphazard manner in which it is used, and the confusion and misunderstanding it often engenders all serve as reasons for the establishment of the term "copyfree" with a clear, standardized definition used to identify licenses that can more properly be understood to permit much more freedom of use than commonly labeled "permissive" licenses.
Probably the closest thing to a clear definition of the term "permissive license" is whatever the current form of the Wikipedia article, Permissive free software licence, says. As of this writing, it reads:
A permissive free software licence is a class of free software licence with minimal requirements about how the software can be redistributed. This is in contrast to copyleft licences, which have reciprocity / share-alike requirements.
This describes the common feeling about what constitutes a permissive license fairly well, including the fact that different people may draw wildly different interpretations of the term's meaning even from this definition, depending on which parts of the definition get the most emphasis. If the key differentiator between licenses that renders one minimal (and another not) is the copyleft "share-alike" conditions of the non-minimal license, this definition allows many more licenses to be classified as permissive than if the lack of copyleft restrictions is only intended as one example among many possible examples. The definition, then, might be interpreted to allow no further restrictions than the bare minimum to call it a license at all.
Most people's interpretations effectively fall somewhere between these extremes, at one end including anything "open source" (or "open culture", or "free culture", or "Free Software") that is not strongly copyleft, and at the other end including only very simple licenses with almost no discernible restrictions at all -- nothing more restrictive than the Revised BSD License and the MIT/X11 license.
Between these extremes lie questions of GPL (or GNU General Public License) compatibility, patent license grants, advertising clauses, DRM prohibitions, weak copyleft characteristics, trademark reservation, and a number of other possibilities. In many cases, a single person's definition of "permissive license" even changes to include or exclude some of these based on circumstance and ignorance, because some people accept certain licenses as permissive without really understanding what the license requires. As such, no real definition of the term exists for some people beyond a label applied haphazardly to whatever licenses the person has heard are permissive, often from conflicting sources.
Supposedly Permissive Licenses
If we are to judge the term "permissive license" by the licenses that are given that label, we might ultimately conclude that it does not have any particular meaning at all. It has essentially been relegated to that category of language inhabited by empty buzzwords. It is generally agreeable to say that BSD licenses and the MIT/X11 License are permissive licenses; beyond those, and very similar licenses, things get a little more difficult to nail down.
Consider the following examples of licenses that are usually called permissive, depending on who you ask:
- Apache License 2.0
- Creative Commons Attribution License
- OpenSSL License
- Original BSD License
- PHP License
- Python Software Foundation License
As you might note with a few minutes of easy research, all of these licenses are on the Copyfree Initiative list of rejected licenses. Reasons for their rejection differ from one license to the next, but include (among other things) bookkeeping requirements, technological prohibitions, and advertising clauses -- the latter of which also tend to render a license GPL-incompatible. These are all examples of license restrictions that either prohibit the use of the code in some circumstances or impose development and distribution requirements outside of the simple acts of modifying the original material to suit your purposes and handing copies to whomever you choose. Some of these restrictions are trivially easy to violate by accident, especially given the tendency many people have to operate on the assumptions about license terms they have heard from others without ever actually reading the license, particularly in the case of longer and more complex so-called "permissive licenses" like the Apache License 2.0 and the Creative Commons Attribution (or CC-BY) License.
The Apache License 2.0 is in fact one of the most problematic licenses generally considered permissive, in part because it is broadly uncontroversial to consider it permissive, and in part because the term "permissive" often gives people an impression of the license terms that is in fact substantively misleading or outright false. Many have the impression that the Apache License 2.0 is essentially just a BSD license with a patent license grant clause or, more colloquially, "patent clause". Because of the complexity of the license language itself -- a necessary consequence of including the terms the Apache Software Foundation requires the license to contain, combined with the need for legal rigor in the language of the license -- the specific terms of the license are not immediately obvious to casual reading, and the length of its legalese text discourages many from actually reading the entire thing. Restrictions that can trivially be accidentally violated in a manner exposing the violator to litigation include requirements for development bookkeeping (i.e. adding notes to altered source files describing the changes made from the original) and maintaining specific files in a project outside of the license file itself. A copyfree license with a patent clause, the Threnodic Ethically Scoped License Agreement (or TESLA), was created specifically to fill the patent clause void left by the recognition that the Apache License 2.0 is not in fact a copyfree license.
The CC-BY License, which many consider a license every bit as permissive as any of the common BSD licenses, is a similarly problematic license. It is even more plagued by dense legalese and lengthy text than the Apache License 2.0, significantly more so in fact, which effectively obscures its somewhat non-permissive terms. The most obvious of these is the key term that disqualifies it for Copyfree Initiative certification, a restriction on technologies that may be used in or with the covered work -- obviously intended as a DRM prohibition, but nonspecific enough in its legal definition that it may well catch other technologies in the crossfire. The attribution requirements of the license also raise the possibility of requiring multiple, redundant forms of attribution that may legally expose the licensee who chooses to redistribute the work without the help of a lawyer in some circumstances of insufficient attribution paranoia.
An example of a "permissive license" that might surprise people is the LGPL, or GNU Lesser General Public License (also known as the Librrary General Public License). Some people call it permissive mostly because, unlike the GPL, it can be directly combined with works using other licenses without the LGPL requiring the entire combined work being distributed under its own terms. Apart from the fact that the legal permissibility of combination with other licenses under terms other than the LGPL has not been comprehensively tested in court, and is thus questionable under some circumstances, there are other reasons to find the LGPL surprising in this context.
The primary reason for such surprise is the fact the LGPL is a copyleft license, though only weakly copyleft as compared with stricter copyleft licenses such as the GPL. Modifications to the original source files, at least, must be redistributed to those recipients of binaries who request the sources. The LGPL is even more restrictive as a copyleft license than some other copyleft licenses, such as the Sleepycat License (and its corporate rebranding, the Open Source License for Berkeley DB). Some people will surely be surprised to find other people who call the LGPL permissive; other people will surely be surprised to find that the LGPL -- which they thought of as permissive -- is in fact a copyleft license.
The Most-Permissive Licenses
Three different approaches to licensing contribute to the concept of a license with maximal permissivity. All example licenses mentioned here are certified by the Copyfree Initiative as conforming to the Copyfree Standard Definition.
It may attempt to release the covered work into the public domain, with a fall-back set of terms intended to mimic the public domain as closely as possible. Examples include the Creative Commons Zero (or CC0) License and the Unlicense. The legal complications of dealing with the concept of the public domain internationally, across jurisdictional variations in the law, including a fallback set of license terms intended to mimic some concept of the public domain, tends to lend itself to complexity of license text whose legal effectiveness may come into question in some areas.
It may simply be a license that allows ad-hoc relicensing or even removal of the license entirely, but without an explicit public domain release. Examples include the Thelemic Crowley License and the WTFPL. The weakness of these license terms may lead to legal enforceability issues in jurisdictions where license grants might be considered valid only if there is some consideration provided by the licensee in exchange for the license grant, though the actual applicability of these concerns is open to question.
It may apply very permissive (but fairly conventional) terms with simplified language similar to the ISC License, MIT/X11 License, or Simplified BSD License, but unlike these examples provide them without specific applicability to software implicit or explicit in the license text. Examples include the Open Works License (or OWL) and the TESLA. A long tradition of software licenses written and employed by developers and corporations whose only relevant concern was the legal status of source code has ensured that the licenses with the longest tenure and strongest mindshare are those that are specific to software in their terms, whether by accident or design; those licenses not specific to any particular type of work tend to be either backed by the organized efforts behind Lawrence Lessig's Creative Commons brand (and usually not copyfree compliant licenses), or of much more recent and less popular vintage.