free software
also: open source, libre software, FOSS
Software that users can freely run, study, modify, and distribute, typically governed by licenses like the GPL that guarantee these freedoms.
Free software is software distributed under licenses that guarantee four core freedoms: the freedom to run the program for any purpose, to study how it works, to modify it, and to share it with others. These freedoms are legally protected, not merely a matter of price.
The most common free software license is the GNU General Public License (GPL), which allows anyone to download, examine, and modify source code. For example, Linux itself is free software—you can inspect the kernel source code, compile your own version with custom changes, and redistribute it under the same license terms.
Free software differs from proprietary software, where the vendor controls the code and restricts modification and redistribution. It also differs from "freeware," which may be free to use but keeps the source code closed. Free software emphasizes freedom and transparency over cost.