An open-source license is a type of license for computer software and other products that allows the source code, blueprint or design to be used, modified and/or shared under defined terms and conditions. This allows end users and commercial companies to review and modify the source code, blueprint or design for their own customization, curiosity or troubleshooting needs. Open-source licensed software is mostly available free of charge, though this does not necessarily have to be the case.
Licenses which only permit non-commercial redistribution or modification of the source code for personal use only are generally not considered as open-source licenses. However, open-source licenses may have some restrictions, particularly regarding the expression of respect to the origin of software, such as a requirement to preserve the name of the authors and a copyright statement within the code, or a requirement to redistribute the licensed software only under the same license (as in a copyleft license). There has previously been debates about whether open source licenses, which permit copyholders to use, transfer, and modify software, have adequate consideration, should be viewed by the courts as legally enforceable contracts. While some academics have argued that open source licenses are not contracts because there is no consideration, others have argued that the significant societal value provided by the role that open source licenses play in promoting software development and improvement by facilitating access to source code offers adequate consideration.
One popular set of open-source software licenses are those approved by the Open Source Initiative (OSI) based on their Open Source Definition (OSD). Open source licenses dictate the terms and conditions that come with the use of open source software (OSS). Open source licenses serve as a legal agreement between open source author and user: authors make OSS available for free, but with certain requirements the user must follow.
Generally, open source license terms kick in upon distribution of your software — if you only use an open source component for an internal tool, for example, you probably won't be bound by requirements that would otherwise apply.
The Free Software Foundation has related but distinct criteria for evaluating whether or not a license qualifies software as free software. Most free software licenses are also considered open-source software licenses. In the same way, the Debian project has its own criteria, the Debian Free Software Guidelines, on which the Open Source Definition is based. In the interpretation of the FSF, open-source license criteria focus on the availability of the source code and the ability to modify and share it, while free software licenses focuses on the user's freedom to use the program, to modify it, and to share it.
Source-available licenses ensure source code availability, but do not necessarily meet the user freedom criteria to be classified as free software or open-source software.
Main article: Public-domain software
See also: Public-domain-equivalent license
Around 2004, lawyer Lawrence Rosen argued in the essay "Why the public domain isn't a license" that software could not truly be waived into the public domain and can't therefore be interpreted as very permissive open-source license, a position which faced opposition by Daniel J. Bernstein and others. In 2012, the dispute was finally resolved when Rosen accepted the CC0 as an open-source license, while admitting that contrary to his previous claims, copyright can be waived away, backed by Ninth Circuit decisions.
The case you referenced in your email, Hampton v. Paramount Pictures, 279 F.2d 100 (9th Cir. Cal. 1960), stands for the proposition that, at least in the Ninth Circuit, a person can indeed abandon his copyrights (counter to what I wrote in my article) -- but it takes the equivalent of a manifest license to do so. :-)[...] For the record, I have already voted +1 to approve the CC0 public domain dedication and fallback license as OSD compliant. I admit that I have argued for years against the "public domain" as an open-source license, but in retrospect, considering the minimal risk to developers and users relying on such software and the evident popularity of that "license", I changed my mind. One can't stand in the way of a fire hose of free public domain software, even if it doesn't come with a better FOSS license that I trust more.