For man-made fusion, the nuclei involved may be two [[deuterium]] (heavy hydrogen) nuclei, each consisting of one [[proton]] and one [[neutron]] and combining to produce a [[helium]] nucleus or [[alpha particle]] (two protons and two neutrons). [[Tritium]] is sometimes also used, but the distinction is not relevant to this article. Other forms of fusion are possible. With hot deuterium fusion, the product is only rarely helium, which is always accompanied by an energetic gamma ray. The vast bulk of such fusions result in a neutron plus Helium-3, or tritium plus a proton.
Since deuterium nuclei have a positive charge, they repel one another and thus the chief problem in achieving ordinary fusion is to get the nuclei close enough together to fuse. With hot fusion, this is accomplished by major heat, millions of degrees.
Cold fusion was given the name, popularly, because the discoverers of cold fusion claimed that the bulk of the heat they found was due to an "unknown nuclear reaction," and most observers thought that it must involve deuterium fusion. Yet the reaction did not produce copious neutrons or tritium, which would be easily detected, contradicting the idea that it was hot fusion somehow occurring under overall cold conditions.