A science fiction interpretation of the sentient soul, containing the will and memory of any living sentient creature in our universe.
An aiùa is, in the science fiction context, an intelligent philote. Sentient creatures consist of uncounted numbers of philotes (a science fiction created indivisible particle) and one aiùa, which holds the collection together and can be thought of as the physical representation of the soul. Bearing in mind that Orson Scott Card is a spiritual person of LDS background, the aiua can be considered the soul in the religious sense as well.
According to Grego in Xenocide, the term was inspired by the Sanskrit word for 'life,' probably "āyus" (this is not the first time Card has derived fictional slang from real-world vocabulary). The existence of philotes and aiùas is also acknowledged, though not as large of a theme, in the parallel Shadow series.
The first time aiúas are mentioned is when Ela, Ender, and Miro are trying to get to the "Outside" a region outside of all universes that contains an infinite number of philotes. In this book, the first in which Ender strays from his usual humanism towards possibly spiritual speculations, Card unites science fiction with an ecumenical religious concept common to all religions as well.