isothermal
ISA is simply an agreement that best expresses the average atmosphere.
Isothermal means "equal temperature (with height)".
The first isothermal region is defined in ISA terms way up at 11-20 kilometres above the ground. It starts at the "tropopause". Way way up beyond that are two more isothermal regions I can think of starting at the stratopause and the mesopause.
So you live in the troposhere where the weather is, above 11km you work in the stratosphere. A few work in the mesosphere. Dan Dare and Digby work beyond that.
But that'll do for now.