1. A reference to nuclear reactions brought about by nuclear fusion (merger or blending); that is, the fusion of hydrogen to helium at temperatures of over 100,000,000° Centigrade.
2. Referring to any process in which a very high temperature is used to bring about the fusion of light nuclei, with the accompanying liberation of energy.

This process is the source of energy of the sun and it is used in the explosion of thermonuclear weapons.