1908, from French stratosphère, literally "sphere of layers," coined by French meteorologist Léon-Philippe Teisserenc de Bort (1855-1913) from Latin stratus "a spreading out" (from past participle stem of sternere "to spread out," from PIE root *stere- "to spread") + French -sphère, as in atmosphère (see sphere).
The region where the temperature increases or remains steady as you go higher. An earlier stratosphere, attested in English 1908 and coined in German 1901, was a geological term for part of the Earth's crust. It is now obsolete. Related: Stratospheric.