Water, hydration and dehydration

Water is an essential nutrient: a multifunctional component of the human body. Water is a:  

Building material

A solvent and a reactant

A carrier

A thermo regulator

Lubricant and shock absorber^[1]

Hydration is about the water levels in something, and dehydration is not enough water. So regarding human health and performance dehydration and poor fluid intake can have a range of negative impacts.

^[1]Kleiner SM. Water: An essential but overlooked nutrient. J Am Diet Assoc 1999:99:201-7