GNU
also: GNU Project, GNU's Not Unix
GNU is a free software project and recursive acronym for 'GNU's Not Unix' that created Unix-like operating system tools and utilities. It forms the core userland of Linux systems.
GNU stands for 'GNU's Not Unix'—a recursive acronym reflecting the project's goal to create a completely free, Unix-compatible operating system without proprietary code. Started by Richard Stallman in 1983, GNU produced essential tools like the C compiler (GCC), shell (Bash), text editors (Emacs), and utilities (coreutils, findutils, grep).
When Linus Torvalds created the Linux kernel in 1991, it was paired with GNU tools to create a complete, functional operating system. This combination is why the full name is often 'GNU/Linux'—Linux provides the kernel, while GNU provides the operating system utilities and tools users interact with daily.
GNU software is distributed under the GPL (General Public License), guaranteeing user freedom to run, modify, and redistribute software. For example, when you type ls, grep, or gcc on a Linux system, you're using GNU tools.