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Open source or open source software, sometimes abbreviated OSS, means any computer software whose source code is either in the public domain or, more commonly, is copyrighted by one or more persons/entities and distributed under an open-source license such as the GNU General Public License (GPL) (This particular license is often refered to as a copyleft). Such a license may require that the source code be distributed along with the software, and that the source code be freely modifiable, with at most minor restrictions, such as a requirement to preserve the authors' names and copyright statement in the code. In some cases, as with Apache or FreeBSD, there are only very minor conditions on use of modified versions. When used as an adjective, the term is hyphenated, e.g. "Apache is open-source software." One common form of open source software uses the OSI Open Source Definition.

These are rights for users of the software. An open-source license itself does not necessarily require that the software, or its source, initially has to be freely (in both senses of the word) available on the Internet. Most popular open-source software is, however.

The term open source in common usage may also refer to any software with publicly available source code, regardless of its license, but this usage provokes strong disapproval from the OSF open source community, which may call them "disclosed source" rather than open source. Examples of such non-OSF open source software include some versions of Solaris and PGP. There are also shared source licenses which, on the surface, appear to be open source, but have critical differences.