Fusion is a nuclear reaction where two or more atonic nuclei combine to form one or more different atomic nuclei and subatomic particles.
Our sun generates energy from the fusion of hydrogen nuclei into helium.
Fusion reactions differ from chemical reactions because they involve changes to one or more nuclei. In the example below a deuterium atom and a tritium atom combine to form a helium atom and a high energy neutron is given off.
Rob Sterling 