The Collaborative International Dictionary of English v.0.48:

Folder \Fold"er\, n.
   One who, or that which, folds; esp., a flat, knifelike
   instrument used for folding paper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

folder
    n 1: a small book usually having a paper cover [syn: booklet,
         brochure, folder, leaflet, pamphlet]
    2: covering that is folded over to protect the contents

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "folder":
   book end, book support, book table, book tray, book truck,
   bookcase, bookholder, booklet, bookrack, bookrest, bookshelf,
   bookstack, bookstand, brochure, chapbook, circular, comic book,
   folio, leaflet, pamphlet, portfolio, revolving bookcase, stack,
   tract



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

directory
directories
folder

    A node in a hierarchical file system which
   contains zero or more other nodes - generally, files or
   other directories.

   The term "folder" is sometimes used in systems such as the
   Macintosh or Microsoft Windows in which directories are
   traditionally depicted as folders (like small briefcases).

   (2007-02-21)