GNU is a complete, free, Unix-compatible operating system started by Richard Stallman and the GNU Project in 1983. Its name is a recursive acronym that, in the project’s own words, means “GNU’s Not Unix - a way of paying tribute to the technical ideas of Unix, while at the same time saying that GNU is something different.” GNU is free software: users are free to run, copy, distribute, study, change, and improve it.
The plan, set out in Stallman’s 1983 announcement and the later GNU Manifesto, was to build a whole system - kernel, compilers, editors, shells, and utilities - so that people could use a computer without proprietary software. Through the 1980s the project produced widely used components: the GNU Emacs editor, the GCC compiler, the GNU debugger, the GNU C library, and the core command-line utilities that still ship on Unix-like systems today.
By the early 1990s most of the system was finished, but its own kernel, GNU Hurd, was not ready. In 1991 Linus Torvalds released the Linux kernel, and the combination of the GNU tools with the Linux kernel produced a complete free operating system. The Free Software Foundation therefore calls the result “GNU/Linux,” reflecting that much of what runs on a typical Linux machine is GNU software.
GNU’s deeper legacy is the model it created: a freely licensed body of system software, protected by copyleft, that anyone can build on. That model underpins much of the open-source software the world now runs.