What does 'Nth ionisation energy' describe?

The term "Nth ionisation energy" describes the energy required to remove an electron from each (n-1)+ ion. This means that as you remove the first electron from a neutral atom, it becomes a positively charged ion with a +1 charge. The first ionisation energy refers to the energy needed to remove that first electron. When considering the second ionisation energy, you are removing an electron from the already positively charged (n-1)+ ion, thus making it an ion with a +2 charge. The trend in ionisation energies shows that they generally increase with each successive removal of an electron because the remaining electrons experience a stronger attraction to the increasingly positive nucleus.

This understanding of ionisation energy is crucial in explaining the reactivity of elements, especially in their ability to form ions. The other options describe different concepts that are not focused on the specific definition of ionisation energy. For example, the addition of electrons pertains to electron affinity rather than ionisation, and descriptions of total energy for shells do not specifically relate to the process of ionisation.