An Etymological Dictionary of Astronomy and Astrophysics

1. The outermost portion of the Earth’s → atmosphere. Extremely tenuous, it lies above the → ionosphere from a height of about 500 km, to the edge of → interplanetary space.
   

2. An extremely tenuous kind of atmosphere surrounding a → solar system body.
   Since the → mean free path is much greater than the atmospheric scale height. The → atoms
   or → molecules never collide with each other. → lunar exosphere.

See also: → exo- + → sphere.