What is the Greek word for heaven?

Ouranos for “sky, heaven”, and pre-Christian and proto-Christian notions of heaven. It’s what “Our father who art in Heaven” uses. And yes, that is the same word as Uranus; Uranus was the sky god.

Once Christianity was entrenched, Heaven as in where the virtuous dead go is Paradeisos, Paradise, as it is in Catholic languages too; English is an anomaly in retaining Heaven for that sense.