Nuclear fission is the splitting of a heavy atomic nucleus into two or more smaller nuclei, releasing a significant amount of energy.
An example is the fission of uranium-235 in nuclear reactors, producing energy and additional neutrons.
Nuclear fusion is the process of combining two light atomic nuclei to form a heavier nucleus, accompanied by a release of energy.
An example is the fusion of hydrogen isotopes (deuterium and tritium) in the sun, generating heat and light energy through a chain reaction.