腹泻（俗称拉肚子，广东俗称肚疴，中医称之为泄泻，英语：diarrhea, diarrhœa 来自希腊语：διάρροια，包含两个字根：「διά /dia ，经过」，以及「ῥέω/rheo，流动」)的定义是在一天之内有软便或水便的状况而且排便三次以上，症状常持续好几天而且可能因为体液流失的关系而产生脱水现象。脱水的初期表现是皮肤失去弹性，而且人格出现变化。如果脱水的情形继续恶化，接下来可能会出现排尿量减少、肤色苍白、心跳过速、或者对外界刺激反应不佳。另外，对以母乳喂养的婴儿来说，软便可能是正常的现象。

Diarrhea, also spelled diarrhoea, is the condition of having at least three loose or liquid bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin with loss of the normal stretchiness of the skin and irritable behaviour. This can progress to decreased urination, loss of skin color, a fast heart rate, and a decrease in responsiveness as it becomes more severe. Loose but non-watery stools in babies who are breastfed, however, may be normal.