Tools that honor the Output Coordinate System environment will create output geodatasets with the specified coordinate system.
Processing (calculation of geometric relationships and modification of geometries) occurs in the same coordinate system as the output geodataset. This environment overrides the default coordinate system on output geodatasets created by geoprocessing tools.
For a description of how the default output coordinate system is determined, see Spatial reference and geoprocessing.
Usage notes
 When the Output Coordinate System environment is different from the input coordinate system, the input is projected to the output coordinate system during tool operation. This projection will not affect the input.
 A projection will not occur if either the input or output coordinate system is unknown. If this is the case, the input's coordinates are assumed to be in the same coordinate system as the output coordinate system.
 When the input and output coordinate system require a geographic transformation, set the Geographic Transformations environment.
Dialog syntax
This specifies the output coordinate system of geodatasets created by geoprocessing tools.
 Current Mapâ€”The coordinate system of the current map or scene will be used.
 All layers are listed, and you can choose one as the coordinate system.
You can also click the Select Coordinate System button to select a different coordinate system.
Scripting syntax
arcpy.env.outputCoordinateSystem = coordinate_system
Parameter  Explanation 

coordinate_system  By default, the coordinate system of an output will be the same as that of the input; otherwise, the Output Coordinate System can be set using any of the following:

Script example
import arcpy
# Set the workspace, outputCoordinateSystem and geographicTransformations
# environments
arcpy.env.workspace = "c:/data"
arcpy.env.outputCoordinateSystem = arcpy.SpatialReference("WGS 1984 UTM Zone 18N")
arcpy.env.geographicTransformations = "Arc_1950_To_WGS_1984_5; PSAD_1956_To_WGS_1984_6"
# Project data into new coordinate system while buffering.
arcpy.analysis.Buffer("roads.shp", "roads_buffer.shp", "10 meters")