What is the iron limit with reference to nuclear fusion?

In nuclear fusion, two atoms are brought together with the
intent of "fusing" the atoms into a single, larger atom. An atom can only have the
number of protons, neutrons, and electrons of its atomic weight, so if two differing
atoms fuse and there is extra mass, it is released in the form of energy or
heat.

The iron limit refers to the maximum weight of a
fusing atom before the process becomes fission rather than fusion. In fission, the two
atoms fusing together require an external supply of energy, and without it, the reaction
will not continue by itself. In stars, for example, all fusion is limited to elements
below the atomic weight of iron, which allows the fusion process to continue by itself;
once iron starts to enter the fusion process, energy is lost and the process starts to
break down, the heavier elements becoming inert and non-reactive. Isotope Iron-56 is the
upper limit of weight that will provide an energetic release without needing an external
energy source.

Simply put, the iron limit is the upper
limit of atomic weight allowed before the fusion process becomes fission, which cannot
be continued without an external source of energy.