What does ūsia mean in Latin?

ūsia, a noun, means “essence, substance”.

What is the genitive of ūsia?

The genitive singular is ūsiae. ūsia is feminine, first declension.

In classical Latin, [ˈuː.si.a]; in ecclesiastical (church) Latin, [ˈuː.s̬i.a].

noun · femininefirst declension

ūsia f (genitive ūsiae)

Classical[ˈuː.si.a]Ecclesiastical[ˈuː.s̬i.a]

Late LatinMedieval Latinessence, substance
Ūsia autem substantia est, id est proprium, quae cēterīs subiacet; reliqua novem accidentia sunt.Usia is substance, proper to the subject, which lies under the others; the remaining nine are accidents.Isidore of Seville, Etymologies 2.26.11

Learned borrowing from Ancient Greek οὐσῐ́ᾱ (ousĭ́ā, “that which is one's own, one's substance”).