folder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that folds or is folded, such as a booklet or pamphlet made of one or more folded sheets of paper.
• noun A flexible cover folded in the center and used as a holder for loose paper.

from The Century Dictionary.

• noun One who or that which folds; specifically, a flat knife-like instrument, frequently of bone or ivory, used in folding paper.
• noun A circular, time-table, map, or other printed paper folded in such a way that it may be spread out in one sheet.
• noun In entomology, one of many insects which fold leaves: as, the grape-leaf folder. See Desmia and leaf-folder.
• noun An attachment to a sewing-machine for bending and folding the fabric previous to sewing. It is made in a great variety of forms to produce the different plaits, folds, and bands used in garment-making and in decorative sewing.
• noun In sheet-metal work, a hand-power machine for folding over the edges of sheet-metal plates to form a lock, turning the edges of small tinware and roofing-plates, etc.: a tin-folding machine.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, folds; esp., a flat, knifelike instrument used for folding paper.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An organizer that papers are kept in, usually with an index tab, to be stored as a single unit in a filing cabinet.
• noun computing A virtual container in a computer's file system, in which files and other folders may be stored. The files and subfolders in a folder are usually related.
• noun A machine or person who folds things.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small book usually having a paper cover
• noun covering that is folded over to protect the contents