Self-referentially, short for GNU's not UNIX, a UNIX-compatible software system developed by the Free Software Foundation (FSF). The philosophy behind GNU is to produce software that is non-proprietary. Anyone can download, modify and redistribute GNU software. The only restriction is that they cannot limit further redistribution. The GNU project was started in 1983 by Richard Stallman at the Massachusetts Institute of Technology.
Linux systems rely heavily on GNU software and in the past, GNU systems used the Linux kernel. This close connection has led some people to mistakenly equate GNU with Linux. They are actually quite separate. In fact, the FSF is developing a new kernel called HURD to replace the Linux kernel in GNU systems.