Question

What is a nuclei fusion?

Answer 1

The joining, usually by a high energy (thus high speed) collision, of the nuclei of two atoms to form a single more massive nucleus. This is sometimes accompanied by the release of neutrons

Answer 2

e.g. \[H_2 + H_3 \rightarrow He_4 + n_1 \]
the high speed collision of a deuterium (hydrogen-2) and a tritium (hydrogen-3) nucleus form helium-4 and a neutron.