Intermolecular force between covalently bound hydrogen atom and atom possessing a lone pair of electrons.
A hydrogen bond is is a special case of a dipole-dipole interaction. The interaction is between a hydrogen atom, bound covalently to more electronegative atom, and another electronegative atom possessing a lone pair of electrons.
Hydrogen bonds can be intermolecular or, for molecules with sufficient size, intramolecular. The molecule or part of a molecule with the bound hydrogen is called the donor, and the atom with the lone pair is called the acceptor.
Hydrogen bonds are weaker than covalent bonds, and they form and break more easily. Like covalent bonds, they are directional, i.e. prefer a certain geometry of donor and acceptor. Networks of hydrogen bonds explain why water, which has two donors and two acceptors per molecule, is a liquid under ambient conditions.