The Collaborative International Dictionary of English v.0.48:

Fiber \Fi"ber\, Fibre \Fi"bre\,, n. [F. fibre, L. fibra.]
   1. One of the delicate, threadlike portions of which the
      tissues of plants and animals are in part constituted; as,
      the fiber of flax or of muscle.
      [1913 Webster]

   2. Any fine, slender thread, or threadlike substance; as, a
      fiber of spun glass; especially, one of the slender
      rootlets of a plant. [WordNet sense 1]
      [1913 Webster]

   3. the inherent complex of attributes that determine a
      person's moral and ethical actions and reactions; sinew;
      strength; toughness; as, a man of real fiber. [WordNet
      sense 2]

   Syn: character, fibre.
        [1913 Webster + WordNet 1.5]

              Yet had no fibers in him, nor no force. --Chapman.
        [1913 Webster]

   4. A general name for the raw material, such as cotton, flax,
      hemp, etc., used in textile manufactures.
      [1913 Webster]

   5. (Nutrition) that portion of food composed of carbohydrates
      which are completely or partly indigestible, such as
      cellulose or pectin; it may be in an insoluble or a
      soluble form. It provides bulk to the solid waste and
      stimulates peristalsis in the intestine. It is found
      especially in grains, fruits, and vegetables. There is
      some medical evidence which indicates that diets high in
      fiber reduce the risk of colon cancer and reduce
      cholesterol levels in the blood. It is also called
      dietary fiber, roughage, or bulk.
      [PJC]

   6. a leatherlike material made by compressing layers of paper
      or cloth. [WordNet sense 3]

   Syn: fibre, vulcanized fiber.
        [WordNet 1.5]

   Fiber gun, a kind of steam gun for converting, wood, straw,
      etc., into fiber. The material is shut up in the gun with
      steam, air, or gas at a very high pressure which is
      afterward relieved suddenly by letting a lid at the muzzle
      fly open, when the rapid expansion separates the fibers.


   Fiber plants (Bot.), plants capable of yielding fiber
      useful in the arts, as hemp, flax, ramie, agave, etc.

WordNet (r) 3.0 (2006):

fiber
    n 1: a slender and greatly elongated substance capable of being
         spun into yarn [syn: fiber, fibre]
    2: coarse, indigestible plant food low in nutrients; its bulk
       stimulates intestinal peristalsis [syn: roughage, fiber]
    3: any of several elongated, threadlike cells (especially a
       muscle fiber or a nerve fiber) [syn: fiber, fibre]
    4: the inherent complex of attributes that determines a persons
       moral and ethical actions and reactions; "education has for
       its object the formation of character"- Herbert Spencer [syn:
       character, fiber, fibre]
    5: a leatherlike material made by compressing layers of paper or
       cloth [syn: fiber, fibre, vulcanized fiber]

Moby Thesaurus II by Grady Ward, 1.0:

126 Moby Thesaurus words for "fiber":
   Aralac, Avisco, Celanese, Chemstrand, Dacron, Dynel, Lastex,
   Manila, Orlon, Terylene, Velon, Vicara, acetate rayon,
   all that lives, alpaca, angora, animal fiber, artificial fiber,
   bast, biosphere, biota, body-build, brand, brawn, capillament,
   cashmere, cast, character, characteristic, characteristics, cilium,
   cirrus, cobweb, coir, complexion, composition, constituents,
   constitution, cotton, crasis, denier, dharma, diathesis,
   disposition, ecosphere, ethos, fabric, fibrilla, filament,
   filamentule, flagellum, flax, flesh, flora and fauna, floss, frame,
   genius, gossamer, grain, habit, hair, hank, hemp, horsehair, hue,
   humor, humors, ilk, jute, kapok, kind, linen, living matter,
   living nature, llama hair, makeup, merino, mohair, mold, nature,
   near-silk, noosphere, nylon, oakum, organic matter, organic nature,
   organized matter, physique, plasm, property, quality, raffia,
   rayon, silk, sisal, skein, somatotype, sort, spandex, spirit,
   spun rayon, stamp, strand, streak, stripe, suchness, suture,
   system, temper, temperament, tendency, tendril, tenor, thread,
   threadlet, tissue, tone, tussah, type, vein, way, web, wool,
   worsted, yarn, zephyr