The combination of two light nuclei into a heavier nucleus, releasing energy because the product has higher binding energy per nucleon. Example: ²_1 H + ³_1 H → ⁴_2 He + ¹_0 n + 17.6 MeV. Fusion requires high temperature and pressure to overcome the Coulomb barrier. Fusion powers stars and is the basis for thermonuclear weapons and experimental fusion reactors.