Dehydration (hypohydration) is defined as excessive loss of body water. It is literally the removal of water (Ancient Greek: ?d??, hÃ½dor) from an object. In physiological terms, it entails a relative deficiency of water molecules in relation to other dissolved solutes. Some definitions even require a rise in blood sodium concentration, but in reality a loss of body water usually accompanies a loss of solutes as well.