The definition of the word Linux depends on the context in which it is used. Linux means the kernel of the system, which is the central controller of everything that happens on the computer.
When most people refer to Linux, they are really referring to a combination of software called GNU/Linux, which defines the operating system. GNU is the free software that provides open-source equivalents of many common UNIX commands. The Linux part of this combination is the Linux kernel, which is the core of the operating system. The kernel is loaded at boot time and stays running to manage every aspect of the functioning system.
The story of Linux begins with UNIX, an operating system developed at AT&T Bell Labs in the 1970s. UNIX is written in the C language making it uniquely portable amongst competing operating systems, which were typically closely tied to the hardware for which they were written. It quickly gained popularity in research and academic settings, as well as amongst programmers who were attracted to its modularity. Over time it was modified and forked (that is, people modified it, and those modifications served as the basis for other systems) such that at present there are many different variants of UNIX. However, UNIX is now both a trademark and a specification, owned by an industry consortium called the Open Group. Only software that has been certified by the Open Group may call itself UNIX.
Linux started in 1991 as a hobby project of Linus Torvalds, a Finnish-born computer scientist studying at the University of Helsinki. Frustrated by the licensing of MINIX, a UNIX-like operating system designed for educational use, and its creator’s desire not to make it a full operating system, Linus decided to create his own OS kernel.
From this humble beginning, Linux has grown to be the dominant operating system on the Internet, and arguably the most important computer program of any kind. Despite adopting all the requirements of the UNIX specification, Linux has not been certified, so Linux really isn’t UNIX! It’s just… UNIX-like.
Prior to and alongside this development was the GNU Project, created by Richard Stallman in 1983. While GNU initially focused on building their own operating system, they ultimately were far more effective at building tools that go along with a UNIX-like operating system, such as the editors, compilers, and user interfaces that make a kernel usable. Since the source was all freely available, Linux programmers were able to incorporate the GNU tools to provide a complete operating system. As such, many of the tools and utilities that are part of the Linux system evolved from these early GNU tools.
Linus originally named the project Freax, however, an administrator of the server where the development files were uploaded renamed it Linux, a portmanteau of Linus’ name and UNIX. The name stuck.
GNU is a recursive acronym for “GNU’s Not Unix,” and it’s pronounced just like the African horned antelope that is its namesake.