GNU

The abbreviation GNU stands for "GNU is Not Unix" and is the name of an open source and free software distribution. The mascot of the GNU operating system is the head of an African wildebeest antelope.

The GNU project initially announced^[1] as an alternative Unix-like environment, served as a collection and tool-set for OS GNU-Hurd (finished 2001) but earlier also for any OS similar to Unix, which wasn't open source from 1981 to 2005. GNU software has been ported to other OS environments too, notably PC-DOS or Windows.
The GNU project was launched in 1983 by the initiator Richard Stallman, who was also the founder of the "Free Software Movement" and in 1985 founder of the "Software Foundation" (FSF) which is still present to this day.

The free available GNU software is used today as GNU/Linux in many distributions. Since the early days Linus Torvalds strictly mandated free software since he started his Linux kernel in 1991 and GNU software (beside many other free software) enriched Linux to a full unix-like environment.