What does "exosphere" mean?

the uppermost layer of a planet's atmosphere

What is a synonym for "exosphere"?

Common synonyms include: outer atmosphere, outer edge, periphery, top of atmosphere.

noun

noun ·Rare ·Advanced level

Noun

1
the uppermost layer of a planet's atmosphere
2
the outermost atmospheric layer wordnet
3
an extremely thin atmosphere, as on Saturn's moon Dione
""Scientists weren't even sure Dione would be big enough to hang on to an exosphere" ('Cassini Detects Hint of Fresh Air at Dione', , JPL, Mar. 02, 2012)"

"The exosphere is the outermost layer of our atmosphere."

From exo- + -sphere.