Final answer:
Microvilli are tiny folds in the plasma membrane of absorptive cells, primarily found in the small intestine, that significantly increase the surface area for nutrient absorption.
Step-by-step explanation:
The numerous tiny folds of the plasma membrane that increase the surface area of certain absorptive cells are called microvilli. These microvilli are cylindrical apical surface extensions of the plasma membrane of the mucosa's epithelial cells, supported by microfilaments. An excellent example of cells with microvilli is those lining the small intestine, where they play a crucial role in the absorption of nutrients from digested food. It is estimated that there are about 200 million microvilli per square millimeter of small intestine. This significant increase in surface area due to microvilli allows for efficient absorption to meet the body's nutritional needs.