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 have the same rights under which it was obtained and that manufacturers of consumer products incorporating free software provide the software as source code. The word free in the term free software refers to freedom (liberty) and is not at all related to monetary cost. The Free Software Foundation advises people to "avoid using terms like 'give away' or 'for free,' because those terms imply that the issue is about price, not freedom." Free software is generally available without charge but is not bound to such a restriction.
Fees are usually charged for distribution on compact discs and bootable USB drives, or for services of installing or maintaining the operation of free software. Development of large, commercially-used free software is often funded by a combination of user donations, corporate contributions, and tax money. The SELinux project at the United States National Security Agency is an example of a federally-funded free software project.
In practice, for software to be distributed as free software, the source code, a human-readable form of the program from which an executable form is produced, must be accessible to the recipient along with a document granting the same rights to free software under which it was published. Such a document is either a free software license or the release of the source code into the public domain.
The free software movement was conceived in 1983 by Richard Stallman to satisfy the need for and to give the benefit of software freedom to computer users. Stallman founded the Free Software Foundation in 1985 to provide the organizational structure to advance his free software ideas.
From 1998 onward, alternative terms for free software came into use. The most common are software libre, free and open source software (FOSS) and free, libre and open source software (FLOSS). The Software Freedom Law Center was founded in 2005 to protect and advance FLOSS. Commercial software may sometimes offer freedoms which are typical of free and open source software. Contrary to a popular misconception that software is either free or commercial, they are unrelated traits, since free software can be commercial and proprietary software can be non-commercial. One example of free commercial software is GNAT, an Ada compiler from the company AdaCore. It has been developed and is available commercially (i.e., against payment), but is free software because of its non-proprietary nature, with the source code publicly available. On the other hand, free software and proprietary software are opposite traits, and an application can be one or the other but never both, contingent upon the availability of the source code under certain minimum freedoms.
Free software, which may or may not be distributed free of charge, is distinct from freeware, which by definition does not require payment for use. The authors or copyright holders of freeware may retain all rights to the software; it is not necessarily permissible to reverse engineer, modify, or redistribute freeware.
Since free software may be freely redistributed, it is generally available at little or no cost. Free software business models are usually based on adding value such as applications, support, training, customization, integration, or certification. At the same time, some business models which work with proprietary software are not compatible with free software, such as those that depend on the user to pay for a license in order to lawfully use the software product.