The abnormal loss of fluid from the body. Water makes up the greater part of the blood and protoplasm, the fundamental material of which the body is composed. Dehydration occurs when less fluid is taken in than is being lost in urine, exhaled breath, perspiration, and stool.

Dehydration often accompanies excessive urination, vomiting, diarrhea, or loss of blood. Fever also causes water loss. Dehydration is sometimes a symptom of diabetes and other diseases. In severe dehydration, saline solutions may be infused. The diarrhea, vomiting, or other condition producing dehydration must also be treated.