GNU , a recursive acronym for GNU's Not Unix, project founded by Richard Stallman to promote free software.
The term GNU's Not Unix is a legal ploy to include the word Unix in the name, thus implying some relationship, while explicitly disclaiming a relationship by the same, potentially avoiding claims of trademark infringement.
GNU refers to various closely related things, but they all stem from Richard Stallman and the FSF's attempt to create a complete UNIX system that is completely free from commercial ties.
Tcl programmers should take note of two particular aspects of the GNU project:
gcc is called nowadays the GNU Compiler Collection, given the variety of front- and backends to the same internal engine.