A microscopic outgrowth from the surface of some tissues and organs, which serves to increase the surface area of the organ. Numerous villi line the interior of the small intestine. Their shape may vary from finger-like (in the duodenum) to spadelike (in the ileum). Intestinal villi are specialized for the absorption of soluble food material: each contains blood vessels and a lymph vessel (see lacteal).

Chorionic villi occur on the chorion of the mammalian placenta, where they increase the surface area for the exchange of materials between the fetal and maternal blood.