Giardia: A genus of protozoa that infect the gastrointestinal tract of some animals, including humans. Giardia have a large sucking disk which permits them to adhere to the intestinal lining. The species that infects humans (and causes diarrhea) is Giardia lamblia.
Named after the French biologist Alfred Mathieu Giard (1846-1908). See also: Giardia lamblia.