mxd = arcpy.mapping.MapDocument("mymap.mxd")
df = mxd.activeDataFrame

newExtent = df.extent
# Set your defined extent here
newExtent.XMin, newExtent.YMin = -180.0, -90.0
newExtent.XMax, newExtent.YMax = 180.0, 90.0

# Zoom to the defined extent
df.extent = newExtent
arcpy.RefreshActiveView()