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

pathname separator

    The character used to separate elements of a
   path or pathname.  Under Unix and POSIX.1 compliant
   systems the pathname separator is the (forward) slash, in
   MS-DOS backslash serves the same purpose.  For obvious
   reasons the no directory or file name can contain this
   character.

   (1996-11-21)