Free software, software libre, or libre software is computer software that gives users the freedom to run the software for any purpose as well as to study, change, and distribute the software and the adapted versions. The right to study and modify free software gives full access to its source code. For computer programs which are covered by copyright law this is achieved with a software license where the author grants users the aforementioned freedoms. Software which is not covered by copyright law, such as software in the public domain is free if the source code is in the public domain (or otherwise available without restrictions). Other legal and technical aspects such as software patents and Digital restrictions management can restrict users in exercising their rights, and thus prevent software from being free. Free software may be developed collaboratively by volunteer computer programmers or by corporations; as part of a commercial, for-profit activity or not.

The free software movement (FSM) or free and open-source software movement (FOSSM) is a social movement with the goal of obtaining and guaranteeing certain freedoms for software users, namely the freedom to run the software, to study and change the software, and to redistribute copies with or without changes. Although drawing on traditions and philosophies among members of the 1970s hacker culture and academia, Richard Stallman formally founded the movement in 1983 by launching the GNU Project.