Source-available software in the context of The Open Source Definition

Proprietary software is software that grants its creator, publisher, or other rightsholder or rightsholder partner a legal monopoly by modern copyright and intellectual property law to exclude the recipient from freely sharing the software or modifying it, and—in some cases, as is the case with some patent-encumbered and EULA-bound software—from making use of the software on their own, thereby restricting their freedoms.

Proprietary software is a subset of non-free software, a term defined in contrast to free and open-source software; non-commercial licenses such as CC BY-NC are not deemed proprietary, but are non-free. Proprietary software may either be closed-source software or source-available software.

Free and open-source software (FOSS) is software available under a license that gives users the right to use, share, modify, and distribute the software – modified or not – to everyone and provides the means to exercise those rights using the software's source code. FOSS is an inclusive umbrella term encompassing free software and open-source software. The rights guaranteed by FOSS originate from the "Four Essential Freedoms" of The Free Software Definition and the criteria of The Open Source Definition. All FOSS can have publicly available source code, but not all source-available software is FOSS. FOSS is the opposite of proprietary software, which is licensed restrictively or has undisclosed source code.

The historical precursor to FOSS was the hobbyist and academic public domain software ecosystem of the 1960s to 1980s. Free and open-source operating systems such as Linux distributions and descendants of BSD are widely used, powering millions of servers, desktops, smartphones, and other devices. Free-software licenses and open-source licenses have been adopted by many software packages. Reasons for using FOSS include decreased software costs, increased security against malware, stability, privacy, opportunities for educational usage, and giving users more control over their own hardware.

The Open Source Definition (OSD) is a policy document published by the Open Source Initiative in 1998. Derived from the Debian Free Software Guidelines written by Bruce Perens, the definition is the most common standard for open-source software. The definition has ten criteria, such as requiring freely accessed source code and granting the open-source rights to everyone who receives a copy of the program. Covering both copyleft and permissive licenses, it is effectively identical to the definition of free software, but motivated by more pragmatic and business-friendly considerations. The Open Source Initiative's board votes on proposals of licenses to certify that they are compliant with the definition, and maintains a list of compliant licenses on its website. The definition has been adapted into the Open Knowledge Foundation's Open Definition for open knowledge and into open hardware definitions.