Define hydrogen bond. How is it originated?

ANS:    The attractive force which binds the hydrogen atom of one molecule with more electronegative atoms like O, S,  N, F of another molecule or same molecule is called hydrogen bond.
If hydrogen atom is linked with more electronegative atoms to form a covalent compound, the shared pair of electrons (bonding electrons) are displaced to the more electronegative atom. The partial displacement of bonding electrons towards the more electronegative atom causes the development of a partial negative negative charge on the atom and a partial positive charge on the hydrogen atom. Thus, the molecule becomes polar.
The electrostatic force of attraction between the positively charged hydrogen atom of one molecule with the negatively charged atom of the neighbouring molecule results in the formation of hydrogen bond.