Quintessence is an overall term for the fifth element. No, not boron; the fifth element in a classical four-element system.
Of course the number of chemical elements is a lot more than four, and there’s no other way to figure matter as being made of four fundamental components either; though it is worth noting that earth, water, air, and fire are one each of the four states of matter; solid, liquid, gas, and plasma.
However, there are four fundamental forces; gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. So when my novel called for a fifth one used to give a scientific-sounding basis to something that resembles “magic”, something just seemed incomplete until I named it aether, after the fifth element that the heavens were made of in Greek philosophy. And the in-story project to develop technology based on this fifth force gave its title to the whole thing, Project Quintessence.
At the same time,the main characters and their mecha have elemental themes, with the fifth one representing space/light/spirit.