Definition of Fusion
When two nuclei with very few nucleons (nucleons are protons or neutrons), such as certain forms of Hydrogen, come very close to one another (due to collisions of great speed, or conditions of very high pressures), they bond to form one nucleus, such as a Helium nucleus. The new nucleus has more nucleons than either of the original ones.
This process is called fusion. Energy is released when the small nuclei join to make larger nuclei.
The new nucleus actually has less mass than the sum of the masses of the original nuclei. So the reaction results in a drop of mass. This change in mass is expressed as energy according to Einstein's famous equation:
E = mc2
The released energy is expressed as heat and light.
Fusion reactions in the core of our Sun are its source of energy.