- The outermost region of a planet's atmosphere.
- The outermost region of the earth's atmosphere, lying above the thermosphere and extending thousands of kilometers into space, from which molecules having sufficient velocity can escape the earth's gravitation.
- the <xref>uppermost</xref> layer of a planet's <xref>atmosphere</xref>
- an extremely thin atmosphere, as on Saturn's moon <xref>Dione</xref>
- the outermost atmospheric layer