In medieval Christian cosmology, the Empyrean was the name given to the outermost part of Heaven, beyond the stars and even beyond the 'first mover' or primum mobile. Since time was thought to be created by the motion of the primum mobile, the Empyrean was conjectured to be outside time - an eternal place where God and other divine beings dwelt.
Now that more is known about outer space, Christians no longer believe in an Empyrean that could be physically reached by simply going in a straight line away from Earth. However, the idea of Heaven (and perhaps Hell) being somewhere in a timeless zone still features in conjectures about God and the afterlife.
The Empyrean has its equivalent in secular cosmology in speculation about what might lie beyond the boundaries of the universe, if such a place could ever be reached.