ingenium

Ingenium, ingenii is a second declension neuter, built from in and gigno: what is born in you. That is the whole idea. It is the raw endowment a person arrives with, set against ars, the skill they acquire, and the pairing ingenium et ars runs through Roman writing on poetry and oratory. Of a place or a thing it means the nature of it - ingenium loci, ingenium arboris.

Two things to watch. It is not "ingenuity" in the narrow sense of cleverness, and it has nothing to do with ingenuus, "freeborn, frank", which merely rhymes. In late and medieval Latin ingenium slid towards "a contrivance, a device", and from there gave French engin and English engine, and, by a longer road, "gin" as in cotton gin.