Now that we have examined the basic principles of copyright and contract and contrasted the operation of those principles with those of open source licensing, it is worth discussing in some detail the definition of open source licensing.
The Open Source Definition is the definition propounded by the Open Source Initiative, used to describe which licenses qualify as "Open Source" licenses. The Open Source Initiative also certifies licenses as OSI Certified to indicate that they fall within the Open Source Definition. We have already seen the basic principles of open source licensing: open source licenses must permit non-exclusive commercial exploitation of the licensed work, must make available the work's source code, and must permit the creation of derivative works from the work itself. Each of these principles is expressed in the Open Source Definition, and, as we will see later, in the open source licenses discussed later in the book.
The Open Source Definition begins as follows:
 The quoted sections are from v1.9 of the Open Source Definition. The definition is frequently updated. Check www.opensource.org for updates.
Open source doesn't just mean access to the source code. The distribution terms of open-source software must comply with the following criteria:
1. Free Redistribution
The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.
This requirement embodies the open distribution principle discussed a moment ago, with the variation that free distribution is required only as part of an "aggregate software distribution." This relatively minor modification of the open distribution principle was made to include the Perl Artistic License described in Chapter 4, under the umbrella of open source. This modification may well be removed in future versions of the Open Source Definition.
2. Source Code
The program must include source code, and must allow distribution in source code as well as compiled form. Where some form of a product is not distributed with source code, there must be a well-publicized means of obtaining the source code for no more than a reasonable reproduction cost-preferably, downloading via the Internet without charge. The source code must be the preferred form in which a programmer would modify the program. Deliberately obfuscated source code is not allowed. Intermediate forms such as the output of a preprocessor or translator are not allowed.
In order to make the open modification principle effective in software, users must have access to source code. The preferred method of distribution is for source code to come with the compiled code. As a general matter, however, distributors prefer to make source code available separately from the compiled code to limit file sizes and ease distribution.
3. Derived Works
The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
This paragraph concisely describes the open modification principle that is fundamental to open source licensing. This paragraph also permits, but does not require, the imposition of a generational limitation (such as copyleft) by the license. As will be made clear in the next chapter, such a generational limitation, even if present in a particular license, may not necessarily bar software from "going closed" being incorporated into proprietary code depending, of course, on the terms of the particular license.
4. Integrity of The Author's Source Code
The license may restrict source-code from being distributed in modified form only if the license allows the distribution of "patch files" with the source code for the purpose of modifying the program at build time. The license must explicitly permit distribution of software built from modified source code. The license may require derived works to carry a different name or version number from the original software.
This is a permissive, not a mandatory, part of the definition. Licenses may limit the open modification principle by requiring distributions of modified source code as original source code plus patches, as described, and still fall within the definition. This license provision allows creators to protect the integrity of their work (and presumably of their reputations) by requiring that modifications be provided and identified as separate from the original work. Such a limitation, however, can apply only to the source code. In order to fall within the definition, the license must permit the free distribution of compiled code as modified, although the license may require a distinct name or number for the modified program.
Because of the logistical complications created by the distribution of source code with patch files, licenses that require such distribution are not recommended.
5. No Discrimination Against Persons or Groups
The license must not discriminate against any person or group of persons.
6. No Discrimination Against Fields of Endeavor
The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
These anti-discrimination provisions ban restrictions on the use or modification of code by selected persons or for particular uses. The motivations behind such restrictions tend to be moral or political: abortion rights activists might oppose the use of their code by those opposed to abortion; oil companies might object to environmental activists using their work, or vice versa. However well-intentioned such restrictions may be, they are antithetical to the notion of open source and, in practice, are damaging to its objectives. Every limitation on the use of a given piece of code restricts the number of potential contributors, and thereby limits the flexibility, reliability, and longevity of that code.
7. Distribution of License
The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.
This requires that licenses have legally effective provisions that give the identical rights to and enforce the generational limitations, if any, on second and subsequent generations of users.
8. License Must Not Be Specific to a Product
The rights attached to the program must not depend on the program's being part of a particular software distribution. If the program is extracted from that distribution and used or distributed within the terms of the program's license, all parties to whom the program is redistributed should have the same rights as those that are granted in conjunction with the original software distribution.
This provision is included to close a loophole under which individual parts of an aggregation of software would be distributed under a different license than the aggregate package, which would be licensed under open source. This loophole allows a fairly obvious end-run around open source principles and is therefore inconsistent with the purposes of open source licensing.
9. The License Must Not Restrict Other Software
The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.
This is not really an open source licensing question at all, but a question of the manner in which software may be distributed. It is included not to directly further the goals of open source but to ensure the freedom of software distributors and to maximize the availability of products licensed under open source licensing.
10. The License must be technology-neutral
No provision of the license may be predicated on any individual technology or style of interface.
This is a housekeeping provision. Some licenses required, as a precaution, that a user take an affirmative action to assent to the license, such as mouse-clicking on a particular box. Because such provisions effectively prohibit the distribution of the program in media (like paper) that are not capable of interpreting acceptance by the user, these licenses effectively limit the free transmission of the code.