「What is "UNIX"? In the narrowest sense, it is a time-sharing operating system kernel: a program that controls the resources of a computer and allocates them among its users. It lets users run their programs; it controls the peripheral devices (discs, terminals, printers, and the like) connected to the machine; and it provides a file system that manages the long-term storage of information such as programs, data, and documents.

In a broader sense, "UNIX" is often taken to include not only the kernel, but also essential programs like compilers, editors, command languages, programs for copying and printing files, and so on.

Still more broadly, "UNIX" may even include programs developed by you or other users to be run on your system, such as tools for document preparation, routines for statistical analysis, and graphics packages.

Which of these uses of the name "UNIX" is correct depends on which level of the system you are considering. When we use "UNIX" in the rest of this book, context should indicate which meaning is implied.」

— Kernighan, Pike, The UNIX Programming Environment