In physiology and medicine, dehydration (hypohydration) is defined as the excessive loss of body fluid, with an accompanying disruption of metabolic processes. It is literally the removal of water (Ancient Greek: hdr) from an object; however, in physiological terms, it entails a deficiency of fluid within an organism. Dehydration of skin and mucous membranes can be called medical dryness. There are three types of dehydration: hypotonic or hyponatremic (primarily a loss of electrolytes, sodium in particular), hypertonic or hypernatremic (primarily a loss of water), and isotonic or isonatremic (equal loss of water and electrolytes). In humans, the most commonly seen type of dehydration by far is isotonic (isonatraemic) dehydration which effectively equates with hypovolemia, but the distinction of isotonic from hypotonic or hypertonic dehydration may be important when treating people who become dehydrated.
Contributions by Syrthiss, Stevenfruitsmaak, and Lee J Haywood.