A collection of files that can be accessed by name and are typically stored on hard disc. Files are chunks of bytes which can be read, written and appended, but typically not inserted.
Relevant aspects:
Special cases:
Experimental or Hypothetical
CP/M, DOS 1.x, and the original
AppleMacintosh didn't have
HierarchicalFileSystems.
The
MainFrame operating system VM/CMS had a non-hierarchical file system. That, plus the rather low limits on filename length, made working with it rather ... interesting.
These are nowadays (2005) considered anachronistic and not-quite file systems. That's probably the reason why
FileSharing systems aren't called file systems. But historically they are and the modern ones are just hierarchical cousins.
See also
FileSystemAlternatives
CategoryFileSystem