An Aiúa is a science fiction interpretation of the sentient soul, containing the will and memory of any living sentient creature in our universe.
Aiùas are explained in the later books of the Ender Quintet, beginning with Xenocide and continuing in Children of the Mind.
An aiùa is, in the science fiction context, an intelligent philote. Sentient creatures consist of uncounted numbers of philotes and one aiùa, which holds the collection together and can be thought of as the physical representation of the soul.
According to Grego Ribeira 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 Saga.
The first time aiúas are mentioned is when Ela Ribiera, Andrew Wiggin, and Miro Ribeira are trying to get to the Outside; a region outside of all universes that contains an infinite number of philotes.