GNU

The GNU Project, founded by Richard Stallman, is largely the source of the concept of Free Software, from which Open Source came (initially due to the need to emphasise that the 'free' in Free Software referred to free access to the source, freedom to modify and freedom to redistribute). See https://www.fsf.org/ for more on Free Software.

The command line environment on a typical Linux distribution is GNU. The 'G' in a lot of Free Software projects (e.g. Gimp, Gnome), stands for GNU.