The name Aetherium comes from the Latin word ether or aether, taken from the English word ethereal, which means "of or relating to the regions beyond the earth" or "suggesting the heavens or heaven."
Before being officialized with a name or flag, it was simply referred to as Calliegender, as a sort of running joke to play on the indescribability of the creators gender identity. It's first proper dedicated conceptualization was in the form of a starter pack image. Several months later, the creator Calliope made and published the official gender and flag, first in its Discord server and then immediately after on their Tiktok.
Upon request, they also created a masculine variation and androgynous variation, Luminium and Alstrium. All genders in this family describe not just a connection to heavenly bodies and the otherworldly as gender, but as feminine, masculine, and androgynous presentations of that gender.