Hydrostatic Gel is a component of the MJOLNIR armor created by the UNSC and its Spartans. It is a blu-ish semi-liquid that forms a layer inside the armor.
Hydrostatic Gel serves several purposes; first, it conforms to the wearer's shape and body temperature, keeping the wearer from getting hot or cold. Second, the Gel can be pressurized to various levels, allowing the wearer an amount of cushioning during hard impacts. This way, a wearer can survive falls from great heights or other similiar conditions as long as they pressurize the armor.
There is an emergency exhaust port in the armor that releases Hydrostatic gel when the suit becomes too hot.