Power

That's what gods are called in Planescape. Well - one of many things that they get called. They're also called gods, puppet-masters, frauds, deities, and divinities. The Athar believe the powers are fakers, little more than mortals who've stumbled on a Dark that most aren't aware of. They're incredibly powerful, sure, but they're too flawed and petty to be worthy of worship. The Athar believe there are things beyond the powers, and a Great+Unknown beyond that which is the True Divine. It is from the Great Unknown that Athar clerics get their spells.

The Believers of the Source agree that there's something beyond the powers, a next stage in divinity. They think the powers are worthy of worship, but they also strive to become them, and to reach the next stage after that, and the next stage after that.

This next step - powers beyond the powers - shouldn't be confused with Overpowers, who are the guardians of a particular crystal sphere on the Prime Material Plane.

There are many classifications of these powers. Some are called demipowers or demigods. Some are called lesser, intermediate, greater, but they all have one thing in common: Power.

Additional Resources:
Planewalker's Godlist