A nutrient is something that provides nourishment to the body. Fiber, while helpful to the body, does not actually provide nourishment, as it simply passes through the body.
Fiber, by definition, cannot be digested by the body. It is the bits of vegetables, fruits and grains that go right through the body instead of being absorbed by the digestive system. Fiber comes in two varieties, the kind that dissolves in water and helps with cholesterol, and the kind that does not dissolve in water and helps move other foods through bodily systems. Fiber also makes the stomach feel more full, which helps to keep one's appetite in check.