light water

Meanings

1. Any water molecules (H₂O) containing two protium (¹H, hydrogen-1) atoms.
2. Water in its usual form (as occurs in nature without human intervention), which is mostly of such form.
3. Water which has had even the usual small amount of deuterium removed (by human intervention): deuterium-depleted water.
4. 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.)

Etymology

By reference to atomic weight and atomic mass; more at light hydrogen.