The innermost self, the will of any living object in our universe.
Aiùas are explained in Orson Scott Card's series of novels, beginning with Speaker for the Dead and continuing in Xenocide, and Children of the Mind. An aiùa is an intelligent philote. Sentient creatures consist of uncounted numbers of philotes (the true indivisible particle) and one aiùa, which holds the collection together and can be thought of as the physical representation of the soul. 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.