What is a nuclear reaction in which two or more atomic nuclei come very close and then collide at a very high speed and join to form a new nucleus called?

A nuclear reaction in which two or more atomic nuclei come very close, collide at high speed, and join to form a new nucleus is called nuclear fusion. This process occurs in stars, including the Sun, where hydrogen nuclei fuse to form helium, releasing vast amounts of energy. Fusion is the source of stellar energy and has potential applications for clean energy on Earth.