Ileus (English pronunciation: /ˈɪliəs/ or /ˈɪlɪəs/) is a disruption of the normal propulsive ability of the gastrointestinal tract.

Ileus is commonly defined simply as bowel obstruction. However, authoritative sources define it as decreased motor activity of the GI tract due to non-mechanical causes. In such sense, this does not include motility disorders that result from structural abnormalities, and, therefore, some mechanical obstructions are misnomers, such as gallstone ileus and meconium ileus, and are not true examples of ileus.

Decreased propulsive ability may be broadly classified as caused either by bowel obstruction or intestinal atony or paralysis. However, there are instances where there are symptoms and signs of a bowel obstruction, but with absence of a mechanical obstruction, mainly in acute colonic pseudoobstruction, also known as Ogilvie's syndrome.

Bowel obstruction is generally a mechanical obstruction of the gastrointestinal tract.

Paralysis of the intestine is often termed paralytic ileus. To be termed "paralytic ileus," the intestinal paralysis need not be complete, but it must be sufficient to prohibit the passage of food through the intestine and lead to intestinal blockage.