Open-source software (OSS) is computer software that is available in source code form: the source code and certain other rights normally reserved for copyright holders are provided under an open-source license that permits users to study, change, improve and at times also to distribute the software.

Open source software is very often developed in a public, collaborative manner. Open-source software is the most prominent example of open-source development and often compared to (technically defined) user-generated content or (legally defined) open content movements.

A report by the Standish Group states that adoption of open-source software models has resulted in savings of about $60 billion per year to consumers.

The free software movement was launched in 1983. In 1998, a group of individuals advocated that the term free software should be replaced by open source software (OSS) as an expression which is less ambiguous and more comfortable for the corporate world. Software developers may want to publish their software with an open source license, so that anybody may also develop the same software or understand its internal functioning. With open source software, generally anyone is allowed to create modifications of it, port it to new operating systems and processor architectures, share it with others or, in some cases, market it. Scholars Casson and Ryan have pointed out several policy-based reasons for adoption of open source, in particular, the heightened value proposition from open source (when compared to most proprietary formats) in the following categories:

In production and development, open source is a philosophy,^{[not in citation given][not in citation given]} or pragmatic methodology^{[not in citation given]} that promotes free redistribution and access to an end product's design and implementation details. Before the phrase open source became widely adopted, developers and producers used a variety of phrases to describe the concept; open source gained hold with the rise of the Internet, and the attendant need for massive retooling of the computing source code.^{[citation needed]} Opening the source code enabled a self-enhancing diversity of production models, communication paths, and interactive communities. The open-source software movement was born to describe the environment that the new copyright, licensing, domain, and consumer issues created.^{[citation needed]}

The open-source model includes the concept of concurrent yet different agendas and differing approaches in production, in contrast with more centralized models of development such as those typically used in commercial software companies. A main principle and practice of open-source software development is peer production by bartering and collaboration, with the end-product, source-material, "blueprints", and documentation available at no cost to the public. This is increasingly being applied in other fields of endeavor, such as biotechnology.

Richard Matthew Stallman (born 16 March 1953), often shortened to rms, is an American software freedom activist and computer programmer. In September 1983, he launched the GNU Project to create a free Unix-like operating system, and he has been the project's lead architect and organizer. With the launch of the GNU Project, he initiated the free software movement; in October 1985 he founded the Free Software Foundation.

Stallman pioneered the concept of copyleft, and he is the main author of several copyleft licenses including the GNU General Public License, the most widely used free software license. Since the mid-1990s, Stallman has spent most of his time advocating for free software, as well as campaigning against software patents, digital rights management, and what he sees as excessive extension of copyright laws. Stallman has also developed a number of pieces of widely used software, including the original Emacs, the GNU Compiler Collection, the GNU Debugger, and various tools in the GNU coreutils. He co-founded the League for Programming Freedom in 1989.