A biosphere is
(1) the part of the earth and its atmosphere in which living organisms exist or that is capable of supporting life;
(2) the living organisms and their environment composing the biosphere.
A Biosphere is the portion of Earth and its atmosphere that can support life.
Other /More definition:
A Biosphere is the living and dead organisms found near the earth's surface in parts of the lithosphere, atmosphere, and hydrosphere. The part of the global carbon cycle that includes living organisms and biogenic organic matter.
Other /More definition:
Biosphere is the zone of the earth where life is found. It consists of parts of the atmosphere (the troposphere), hydrosphere (mostly surface water and groundwater), and lithosphere (mostly soil and surface rocks and sediments on the bottoms of oceans and other bodies of water) where life is found. Sometimes called the ecosphere.