Nuclear fusion is the process in which two atomic nuclei are fused together creating a more massive nucleus and expelling energy. when nuclei fuse together, the mass of the new nucleus will be less than that of the combined mass of the parent nuclei, the difference in mass is called the mass deficit

Alpha Radiation

Thermonuclear Fusion in stars (Stellar Nucelosynthesis)

In the context of a star, high temperatures - fusion generates the stars energy