Definition for Nuclear fusion
From Biology Forums Dictionary
1. In nuclear physics, nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at very high speed and join to form a new type of atomic nucleus (e.g. The energy that the Sun emits into space is produced by nuclear reactions that happen in its core due to the collision of hydrogen nuclei and the formation of helium nuclei). During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to photons (energy). Fusion is the process that powers active or "main sequence" stars.
2. Forcing together the small nuclei of lightweight elements under extremely high temperature and pressure.