- A liquid made up of molecules of hydrogen and oxygen (HO2). When pure, it is colorless, tasteless, and odorless. It exists in gaseous, liquid, and solid forms; it is liquid at room temperature. It is the liquid of which seas, lakes, and rivers are composed, and which falls as rain. Water is one of the most plentiful and essential of compounds. It is vital to life, participating in virtually every process that occurs in plants and animals. One of its most important properties is its ability to dissolve many other substances. The versatility of water as a solvent is essential to living organisms. The term "water" is typically used to refer to the liquid form of this compound; for the solid or gaseous forms, use "ice" or "water vapor."