Constipation, costiveness, or irregularity, is a condition of the digestive system in which a person (or animal) experiences hard feces that are difficult to expel. This usually happens because the colon absorbs too much water from the food. If the food moves through the gastro-intestinal tract too slowly, the colon may absorb too much water, resulting in feces that are dry and hard. Defecation may be extremely painful, and in severe cases (fecal impaction) lead to symptoms of bowel obstruction. The term obstipation is used for severe constipation that prevents passage of both stools and gas. Causes of constipation may be dietary, hormonal, anatomical, a side effect of medications (e.g. some opiates), or an illness or disorder. Treatments consist of changes in dietary and exercise habits, the use of laxatives, and other medical interventions depending on the underlying cause.

In common constipation, the stool is hard, difficult, and painful to pass. Usually, there is an infrequent urge to void. Straining to pass stool may cause hemorrhoids. In later stages of constipation, the abdomen may become distended and diffusely tender and crampy, occasionally with enhanced bowel sounds.

The definition of constipation includes the following:

• infrequent bowel movements (typically three times or fewer per week)
• difficulty during defecation (straining during more than 25% of bowel movements or a subjective sensation of hard stools), or
• the sensation of incomplete bowel evacuation.

Severe cases ("fecal impaction") may feature symptoms of bowel obstruction (vomiting, very tender abdomen) and "paradoxical diarrhea", where soft stool from the small intestine bypasses the impacted matter in the colon.

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