Terminology
The water molecule has the chemical formula H2O, meaning each molecule of water is composed of two hydrogen atoms and one oxygen atom. Literally, the term "dihydrogen monoxide" means "two hydrogen, one oxygen", consistent with its molecular formula: the prefix di- in dihydrogen means "two", the prefix mon- in monoxide means "one", and an oxide is a compound that contains one or more oxygen atoms.
Various names for water are commonly used within the scientific community. Some such names include hydrogen oxide, as well as an alkali name of hydrogen hydroxide, and several acid names such as hydric acid, hydroxic acid, hydroxyl acid, and hydroxilic acid. The term "hydroxyl acid" used in the original hoax is a non-standard name. An additional name, μ-oxido dihydrogen, has been developed for this compound.
Under the 2005 revisions of IUPAC nomenclature of inorganic chemistry, there is no single correct name for every compound. The primary function of chemical nomenclature is to ensure that each name refers, unambiguously, to a single substance. It is considered less important to ensure that each substance should have a single name, although the number of acceptable names is limited. Water is one acceptable name for this compound, even though it is neither a systematic nor an international name and is specific to just one phase of the compound. The other IUPAC recommendation is oxidane.
The use of numerical prefixes is typical nomenclature for compounds formed by covalent bonds, which are present in water. The prefix for the first named element is often dropped if the elements involved commonly form only one compound, or even if the number of atoms of the first-named element is the same in all the compounds of the two (or more) elements. Thus H2S is often simply called hydrogen sulfide, and lithium oxide is a common name for Li2O. However, the names dihydrogen sulfide, dilithium oxide, and dilithium monoxide are also commonly used both in industry and in universities, despite the fact that Li2O is ionic.
The mono- prefix is often dropped for the second-named element if it is the only common compound the elements form. Thus for instance the IUPAC name of H2S is hydrogen sulfide rather than hydrogen monosulfide. However, since carbon and oxygen can form several compounds (carbon monoxide, carbon dioxide, tricarbon dioxide, and dicarbon monoxide), the mono- prefix is kept, as it is with silicon monoxide and silicon dioxide. Indeed, hydrogen and oxygen do form another common compound, H2O2 (hydrogen peroxide). Using prefix nomenclature, H2O2 would be called dihydrogen dioxide. Thus, keeping the mono- in dihydrogen monoxide does serve to distinguish it from another compound.
Read more about this topic: Dihydrogen Monoxide Hoax