Also called physiological atmosphere. the part of the atmosphere in which it is possible to breathe normally without aid: the portion of the troposphere from sea level to an altitude of about 13,000 feet (4000 meters).
Ecology. the planetary ecosystem, including all the earth's living organisms and their physical environment; biosphere.
1953, the region around a star where conditions allow life-bearing planets to exist; see eco- + sphere. Apparently coined by German-born U.S. physician and space medicine pioneer Hubertus Strughold (1898-1986).