An
isothermal layer refers to a region or layer in the atmosphere where the temperature remains relatively constant. This means that there is little or no change in temperature as you move through that particular layer.
Full definition
Likewise the stupid explanation by realclimate starts out with «Imagine an atmosphere with
multiple isothermal layers that only interact radiatively.»
Above the tropopause and
the isothermal layer in the lower stratosphere, temperature increases with height.