Typically, the word "Linux" is used to refer to a complete Linux distribution and all the cooperating programs that make the distribution work. However, you may be surprised to find out that, technically, Linux is a kernel, and a kernel only. While the other parts of what we commonly call "Linux" (such as a shell and compiler) are essential parts of a complete operating environment, they are technically separate from Linux (the kernel). Still, people will continue to use the word "Linux" to mean "Linux-based distribution." Nevertheless, everyone can at least agree that the Linux kernel is the heart of every "Linux OS."