  The concept of compound nucleus was proposed by Niels Bohr in 1936 to explain
narrow resonances observed in scattering of a slow neutron off atomic nuclei. A
compound nucleus is a metastable state with a long lifetime, in which all the
degrees of freedom are in a sort of thermal equilibrium. Fusion reactions are
defined as reactions to form such compound nucleus by merging two atomic
nuclei. Here a short description of heavy-ion fusion reactions at energies
close the Coulomb barrier is presented. This includes: (i) an overview of a
fusion process, (ii) a strong interplay between nuclear structure and fusion,
(iii) fusion and multi-dimensional/multi-particle quantum tunneling, and (iv)
fusion for superheavy elements.
