https://doi.org/10.1351/goldbook.C01331
  1. The @[email protected] number of a specified atom in a @[email protected] is the number of other atoms directly linked to that specified atom. For example, the @[email protected] number of carbon in methane is four, and it is five in protonated methane, CH5+. (The term is used in a different sense in the crystallographic description of ionic crystals.)
    Source:
    PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1100 [Terms] [Paper]
  2. In an inorganic @[email protected], the number of σ-bonds between @[email protected] and the @[email protected] π-bonds are not considered in determining the @[email protected] number.
    Source:
    Red Book, 3rd ed., p. 146 [Terms] [Book]