The first definition of water in the dictionary is a clear colourless tasteless odourless liquid that is essential for plant and animal life and constitutes, in impure form, rain, oceans, rivers, lakes, etc. It is a neutral substance, an effective solvent for many compounds, and is used as a standard for many physical properties. Formula: H2O related adjective aqueous related combining forms hydro- aqua-. Other definition of water is any body or area of this liquid, such as a sea, lake, river, etc. Water is also the surface of such a body or area.