A slender, elongated, threadlike object or structure. Botany One of the elongated, thick-walled cells that give strength and support to plant tissue. Anatomy Any of the filaments constituting the extracellular matrix of connective tissue. Anatomy Any of various elongated cells or threadlike structures, especially a muscle fiber or a nerve fiber. A natural or synthetic filament, as of cotton or nylon, capable of being spun into yarn. Material made of such filaments. Something that provides substance or texture. Essential character: "stirred the deeper fibers of my nature\u201d ( Oscar Wilde). Basic strength or toughness; fortitude: lacking in moral fiber. Coarse, indigestible plant matter, consisting primarily of polysaccharides such as cellulose, that when eaten stimulates intestinal peristalsis. Also called bulk, roughage.