zero population growth (ZPG) State in which the birth rate (plus immigration) equals the death rate (plus emigration) so the population of a geographic area is no longer increasing. 

zone of aeration Zone in soil that is not saturated with water and lies above the water table. See water table, zone of saturation. 

zone of saturation Area where all available pores in soil and rock in the earth's crust are filled by water. See water table, zone of aeration.

zoning Regulating how various parcels of land can be used.