Define hydrogen bond ?

A hydrogen bond is a type of attractive (dipole-dipole) interaction between an electronegative atom (especially a nitrogen, oxygen, or flourine atom) and a hydrogen atom bonded to another electronegative atom. This bond always involves a hydrogen atom. Hydrogen bonds can occur between molecules or within parts of a single molecule. A hydrogen bond tends to be stronger than van der Waals forces, but weaker than covalent bonds or ionic bonds.