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 for exothermic reactions. For endothermic reactions, energy is absorbed (products heavier than reactants).
Remarks
I personally find this aspect of nuclear fusion reactions awesome. This is like alchemy, where you are literally 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 exhaustive, impractical and not a commercially attractive alternative to mining.
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.
On definitions and multiple reactants
Definitions Atomic Nucleus
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 half life od 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
Comparison to chemical reactions: In comparison to a chemical reaction, which involves redistribution of electrons with no change to the primary element atoms, the difference in nuclear reaction is that it involves transformation of these elements (or at least transformation of a nuclide). The fundamental force involved is electromagnetic in chemical reactions while it is nuclear in nuclear reactions (much stronger, but acts over short range). This is responsible for the difference in energies involved typically differing by orders of magnitude.
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."
Caveats
Source:It's worth noting that while most definitions focus on light atom nuclei as reactants, fusion with heavier atoms are not necessarily impossible, but they are not helpful for energy release for reasons that will be covered in the section on Binding Energy.
The nucleons in an atom are held together (or bound together) due to “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 femtometre, or 10-15 metre, which is the order of magnitude of nuclear diameters.
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.