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
[Technical digression: Imagine an atmosphere with multiple isothermal layers that only interact radiatively. (realclimate.org)
That's the thing about finite element analysis models, they have unphysical isothermal layers. (scienceofdoom.com)
Likewise the stupid explanation by realclimate starts out with «Imagine an atmosphere with multiple isothermal layers that only interact radiatively.» (wattsupwiththat.com)