What is the Exosphere in the Atmosphere?

In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space. It is located directly above the thermosphere.

"The exosphere is the outermost area of a planet's atmosphere. It is made up of mostly hydrogen, with some amounts of helium, carbon dioxide, and atomic oxygen."