Pronunciation: (ek'ō-sfēr", ē'kō-), [key]
1. 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 ft. (4000 m).
2. Ecol.the planetary ecosystem, including all the earth's living organisms and their physical environment; biosphere.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.