H₂O

Symbol

 * 1)  water (even if not chemically pure)
 * 2)  (a molecule of) water
 * 1)  (a molecule of) water

Usage notes
Deuterium should be called by the chemical symbol "$2$H", not "D". Therefore, even heavy water (often called D$2$O) should properly be called H$2$O; $2$H$2$O to be precise. Likewise, super-heavy water should be called $3$H$2$O, not T$2$O.

Derived terms

 * H₃O+, H₃O, hydronium
 * OH-, OH, hydroxide

Etymology
Two H initials (Hishammuddin Hussein) represented by H₂ + O for Onn, chosen to humorously match the chemical formula for water.

Proper noun

 * 1)  Hishammuddin Hussein Onn, Malaysian politician and son of Hussein Onn, third prime minister of Malaysia.