1. The final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum. (wikipedia.org)
2. The last, and usually the longest, division of the small intestine; the part between the jejunum and large intestine. (wiktionary.org)
3. The terminal (or distal) portion of the small intestine, characterized by narrower diameter, numerous Peyer’s patches, fewer circular folds, and smaller, rounder villi on the surface for adsorption of enzymes and absorption of digestive products. (biology-online.org)
Word origin: Medical Latin, from ileum, singular created from classical Latin plural ilia “groin, flank,” in classical Latin, “belly, the abdomen below the ribs,” poetically, “entrails, guts.” Sense restriction and form apparently from confusion with Greek eileos. Earlier in English ylioun (late 14c.), from Medieval Latin ileon.