Commercial Open Source Software
Encyclopedia
The phrase Commercial open source software (COSS) is used as a euphemism for Proprietary Open Source Software that contains some elements of free and open source software
Free and open source software
Free and open-source software or free/libre/open-source software is software that is liberally licensed to grant users the right to use, study, change, and improve its design through the availability of its source code...

 in order to legitimately claim to be "open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...

", however sometimes also limits availability of some generally enhanced functionality to proprietary software
Proprietary software
Proprietary software is computer software licensed under exclusive legal right of the copyright holder. The licensee is given the right to use the software under certain conditions, while restricted from other uses, such as modification, further distribution, or reverse engineering.Complementary...

 which it sells under a closed and limited license . The release of some code under closed licensing builds in a potential path back to vendor lock-in
Vendor lock-in
In economics, vendor lock-in, also known as proprietary lock-in or customer lock-in, makes a customer dependent on a vendor for products and services, unable to use another vendor without substantial switching costs...

 if the functionality is ever required, and so the overall software is not "free" in the original sense and so does not qualify as FOSS as a whole. Arguably the right term for these "mixed-source" applications is Proprietary Software, while the term many of the companies behind them prefer, is COSS.

True Commercial Open Source Software

Note that software licensing, the legal language heading each software module, is completely separate from how the software is used. Since its beginning as free software
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 and the GPL License, FOSS has been legally able to be sold, for example in binary with regular patching, or with support agreements including rapid response, or with service contracts to match configuration to an organization's process workflow. As long as the license requires the source be made freely available, the software remains FOSS. Indeed, the primary product of Red Hat
Red Hat
Red Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide....

, the most commercially successful open source company, remains a distribution packaging of the FOSS operating system Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

. Therefore, the term COSS refers not to how software is used, but to a certain type of software that includes some free open source software and some software elements released under closed proprietary licensing.

Dual Licensing

Dual-licensed software is a different issue, and refers to free open source software that is also released by its authors under a proprietary license. This was done sometimes in the past for administrative reasons, for particular customers that preferred proprietary licensing. However, as long as all the software is also available under an open license approved by the OSI
Open Source Initiative
The Open Source Initiative is an organization dedicated to promoting open source software.The organization was founded in February 1998, by Bruce Perens and Eric S. Raymond, prompted by Netscape Communications Corporation publishing the source code for its flagship Netscape Communicator product...

then it is still FOSS. The practical problem with dual-licensing is only copyright owners can do it, so unless a company somehow holds the copyright of every single author, it is usually practically impossible to get permission to dual-license.
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