HBr is the chemical symbol for hydrogen bromide, and H2O is the chemical symbol for water. The combination of the two is referred to as HBr H2O.
When not dissolved in water, hydrogen bromide is called anhydrous hydrogen bromide. It is a colorless gas with a sharp, irritating odor. When it is mixed with water, it dissolves to form hydrobromic acid, which consists of bromide ions (Br-) in a solution of H2O and H3O+. Because of its propensity to react with any available water to form a strong acid, anhydrous hydrogen bromide is a very corrosive and dangerous substance, and it is usually stored and shipped as a liquefied compressed gas.