Fusion Reactions
Nuclear Reactions
A nuclear reaction is a process that results in a transformation of an atomic nucleus (or nuclei). The process is initiated with reactants consisting nuclei (at least one) and may also include subatomic particle(s). The mass difference between the reactants and the products is balanced by energy as per mass-energy equivalence relationship.
For a typical reaction considered for energy release, we have: $$Reactants \rightarrow Products + Energy $$
Where, the energy released is related to mass difference as (Note that 'c' denotes speed of light):
$${Energy} = \left({Mass}_{Products} - {Mass}_{Reactants}\right)\cdot c^2 $$
The release of energy only applies to exothermic reactions. For endothermic reactions, energy is absorbed (products are heavier than reactants).
Remarks
I personally find this aspect of nuclear fusion reactions to be awesome. It feels very much like science fiction in our lives, given how so much of our daily life involves noticeable chemical reactions instead of nuclear reactions. This is like alchemy, where you are changing one material to another at an elemental level. Yes, it's physically possible to accomplish the "Philosopher's stone" style conversion of base metals into gold. However, it'd be very energy-intensive, impractical and not a commercially attractive alternative to mining, as it stands today.
On definitions and multiple reactants
Definitions Atomic Nucleus
The "number of reactants" considered is usually restricted to two since reactions involving more reactants are probably rare. Furthermore, it may be inconsequential if it can be treated as temporally successive reactions, apart from some constraints on the time (such as the half life of the intermediate product) between the reactions. An example of such a reaction is the triple alpha process.
Energy-momentum relation governs the relativistic energy for a system in motion (a generalised form of per mass-energy equivalence relationship).
Comparison to chemical reactions
In comparison to a chemical reaction, which involves the redistribution of electrons with no change to the primary element atoms, the difference in nuclear reaction is that it involves the transformation of these elements (or at least the transformation of nuclei). The fundamental force involved is "electromagnetic" in chemical reactions, while it is "nuclear" in nuclear reactions (much stronger, but acts over a short range). This is responsible for the typical energies involved in reactions being orders of magnitude different.
Nuclear Fusion Reaction
"Nuclear fusion is the process by which two light atomic nuclei combine to form a single heavier one while releasing massive amounts of energy."
References and Notes
It's worth noting that while most definitions focus on light atom nuclei as reactants, fusion with heavier atoms is not necessarily impossible. However, they are not helpful for energy release. This is due to reasons that will be covered in the section on Binding Energy.
Source:The nucleons in an atom are held together (or "bound" together) due to the “strong nuclear force”. These forces act in a non-linear fashion compared to electromagnetic or gravitational forces. In the context of nuclear reactions, these forces dominate when the reactants are in close proximity. This is in the order of a femtometre, or 10-15 metre, which is the order of magnitude of the diameter of a nucleus.
For nuclear reactants that are electrically charged ("positive", if involving protons in nuclei, as is the case for nuclear fusion reactions), this poses a problem, as the electromagnetic forces lead to a repulsive force that needs to be overcome. This, as explained later, is one of the great challenges to initiating fusion reactions.
Other ways to overcome electromagnetic force
There are other nuclear reactions, like "neutron capture", that involves a neutral reactant such that this repulsive electromagnetic force is not a concern.
It's also worth noting that the phenomenon of quantum mechanical tunnelling helps with overcoming the electromagnetic force barriers in achieving thermonuclear fusion in stars.