What is the X in X-mas?
1630s, atmosphaera (modern form from 1670s), from Modern Latin atmosphaera, from atmo-, comb. form of Greek atmos "vapor, steam" + spharia "sphere" (see sphere). Greek atmos is from PIE *awet-mo-, from root *wet- "to blow, inspire, spiritually arouse" (see wood (adj.)). First used in English in connection with the Moon, which, as it turns out, doesn't have one.
It is observed in the solary eclipses, that there is sometimes a great trepidation about the body of the moon, from which we may likewise argue an atmosphaera, since we cannot well conceive what so probable a cause there should be of such an appearance as this, Quod radii solares a vaporibus lunam ambitntibus fuerint intercisi, that the sun-beams were broken and refracted by the vapours that encompassed the moon. [Rev. John Wilkins, "Discovery of New World or Discourse tending to prove that it probable there may be another World in the Moon," 1638]Figurative sense of "surrounding influence, mental or moral environment" is c.1800.
atmosphere at·mos·phere (āt'mə-sfǐr')
A gas surrounding a given body; a gaseous medium.
Abbr. atm, atm. A unit of pressure equal to the air pressure at sea level, approximately equal to 1.01325 × 105 newtons per square meter.