Free and open-source software (F/OSS, FOSS) or free/libre/open-source software (FLOSS) is software that is both free software and open source. It is liberally licensed to grant users the right to use, copy, study, change, and improve its design through the availability of its source code. This approach has gained both momentum and acceptance as the potential benefits^{[clarification needed]} have been increasingly recognized by both individuals and corporations.

In the context of free and open-source software, free refers to the freedom to copy and re-use the software, rather than to the price of the software. The Free Software Foundation, an organization that advocates the free software model, suggests that, to understand the concept, one should "think of free as in free speech, not as in free beer".

FOSS is an inclusive term that covers both free software and open source software, which despite describing similar development models, have differing cultures and philosophies. Free software focuses on the philosophical freedoms it gives to users, whereas open source software focuses on the perceived strengths of its peer-to-peer development model. FOSS is a term that can be used without particular bias towards either political approach.

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:

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.