Fusion is when two particles collide to form one bigger particle.
When the two light nuclei fuse to form a heavier nucleus, the new mass is less than that of the two components. It is when an atomic nucleus divides into two smaller nuclei. The two smaller nuclei formed in this way have a combined mass less than the original mass of the nucleus. The combined mass of the fragments of fission nucleus is less than the original. He discovered a world of negative energy that was diametrically opposed to the conventional understandings of matter and energy.