light water

noun7

1. •
   Water in its usual form (as occurs in nature without human intervention), which is mostly of such form.
2. •
   Normal water, as opposed to heavy water.
3. •
   Any water molecules (H₂O) containing two protium (¹H, hydrogen-1) atoms.
4. •
   Fire fighting foam.

   • Also, once you use light-water on a fuel fire, it's nearly impossible to get it to re-light for subsequent firefighters. — 2016
5. •
   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.)

   • The light water (AFFF) of the TACR was to be backed up by fluorinated protein foam instead of standard protein foam. — 2001
6. •
   Water which has had even the usual small amount of deuterium removed (by human intervention): deuterium-depleted water.
7. •
   Alternative form of light water.