What is "transition region"

Usage examples of "transition region".

He had reached the transition region where the blue of ground level is being overtaken by the perfect black of space.

There was a solid rocky core to the planet, a spherical mass of about ten Earth-sized planets buried under seventy thousand vertical kilometres of hydrogen, helium and ice, and there were purists who would call the transition region between that stony kernel and the high-temperature, high-pressure water ice above it the planet's surface.