The name for two or more elements that are bonded to each other chemically is a compound. Compounds are not to be confused with isotopes or mixtures. Isotopes are alternate forms of an element's atoms, and mixtures are combinations of chemicals that are joined physically but not chemically.
Water is one of the of the most abundant compounds on Earth. It is formed from the chemical bonding of one oxygen atom to two atoms of hydrogen. Unlike a mixture or an emulsion, in which the molecules of a substance are suspended in proximity to the molecules of another substance, water is a true compound in that its constituent elements form covalent bonds with each other and share electron pairs. The fact that water is a compound can be seen in the way both hydrogen and oxygen are chemically altered by forming it. Oxygen and hydrogen, by themselves, are each flammable gases. When they are brought together, however, the chemical change between them produces a liquid that can be used to douse fires. This change in properties is what sets a true compound apart from mixtures. Mixtures do not chemically alter their constituent substances, all of which retain their distinctive traits.