light water

Short definition from Wiktionary

Noun

1. 1
   Any water molecules (H₂O) containing two protium (¹H, hydrogen-1) atoms: Water in its usual form (as occurs in nature without human intervention), which is mostly of such form.
2. 2
   Any water molecules (H₂O) containing two protium (¹H, hydrogen-1) atoms: Water which has had even the usual small amount of deuterium removed (by human intervention): deuterium-depleted water.
3. 3
   Foam formed by water and any of various additives, used in firefighting because it floats on flammable liquids lighter than water. (The additives were originally fluorocarbon surfactants but today are increasingly fluorine-free.)
4. 4
   Alternative form of light water: Normal water, as opposed to heavy water.
5. 5
   Alternative form of light water: Fire fighting foam.