A hydrogen bond is an interaction wherein a hydrogen atom is attracted to two atoms, rather than just one, and acts like a bridge between them. The strength of this attraction increases with the increasing electronegativity of either of the atoms, and in the classical view, all hydrogen bonds are highly electrostatic and sometimes even partly covalent. Gradually, the concept of a hydrogen bond has become more relaxed to include weaker and more dispersive interactions, provided some electrostatic character remains. A great variety of very strong, strong, moderately strong, weak, and very weak hydrogen bonds are observed in practice. Weak hydrogen bonds are now invoked in several matters in structural chemistry and biology. While strong hydrogen bonds are easily covered by all existing definitions of the phenomenon, the weaker ones may pose a challenge with regard to nomenclature and definitions. Recently, a recommendation has been made to the International Union of Pure and Applied Chemistry (IUPAC) suggesting an updated definition of the term hydrogen bond. This definition will be discussed in greater detail.