The following are definitions given to dietary Fiber by various authors.

Plant substances not digested by human digestive enzymes, including plant cell wall substances (cellulose, hemicellulose, pectin and lignin) as well as the intercellular polysaccharides such as gums and mucilages. Largely identical to undigested (unavailable) carbohydrates plus lignin.
(Trowell, H.)
The sum of lignin and the polysaccharides that are not hydrolyzed by the indigenous secretions of the human digestive tract.
(Southgate, D. A. T.)
Chemical definition: the sum of the plant nonstarch polysaccharides and lignin. Physiological definition: the remnant of plant foods resistant to hydrolysis by the elementary enzymes of humans.
(Furda, I.)
Dietary fiber is the edible plant or animal material not hydrolyzed by the endogenous enzymes of the human digestive tract as determined by the agreed upon method.
(FAO/WHO, 1995 (Codex Alimentarius Commission))
Dietary fiber is the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant substances. Dietary fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation, and/or blood glucose attenuation.
(American Association of Cereal Chemists (AACC), 2000)
Dietary fiber is that fraction of the edible part of plants or their extracts, or analogous carbohydrates, that are resistant to digestion and absorption in the human small intestine, usually with complete or partial fermentation in the large intestine. The term includes polysaccharides, oligosaccharides (degrees of polymerization >2), and lignins. Dietary fibre promotes one or more of these beneficial physiological effects: laxation, reduction in blood cholesterol, and/or modulation of blood glucose.
(Australia New Zealand Food Authority (ANZFA) (Proposed), 2000)

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Spiller, G. A. 2001. Definitions of Dietary Fiber: In, CRC Handbook of Dietary Fiber in Human Nutrition. Ed. Gene A. Spiller. 3rd ed. CRC press, Boca Raton, pp. 709.

Institute of Medicine. 2001. Dietary Reference Intakes: Proposed Definition of Dietary Fiber. National Academy Press, pp. 74.