WordNet (r) 3.0 (2006):

file system
    n 1: a system of classifying into files (usually arranged
         alphabetically) [syn: file system, filing system]

The Free On-line Dictionary of Computing (30 December 2018):

file system
filing system

    (FS, or "filesystem") 1. A system for
   organizing directories and files, generally in terms of
   how it is implemented in the disk operating system.  E.g.,
   "The Macintosh file system is just dandy as long as you
   don't have to interface it with any other file systems".

   2. The collection of files and directories stored on a given
   drive (floppy drive, hard drive, disk partition, logical
   drive, RAM drive, etc.).  E.g., "mount attaches a named file
   system to the file system hierarchy at the pathname location
   directory [...]" -- Unix manual page for "mount(8)".

   As an extension of this sense, "file system" is sometimes used
   to refer to the representatation of the file system's
   organisation (e.g. its file allocation table) as opposed the
   actual content of the files in the file system.

   Unix manual page: fs(5), mount(8).

   (1997-04-10)