From AMS Glossary
At least one coordinate of each fluid parcel must therefore be unvarying with time. Such a system takes advantage of the fact that in many atmospheric models there is one property (but not three properties) conserved in the motion. The most frequently used system of this type is the (x, y, θ) system under adiabatic motions, where x and y are Cartesian coordinates, and θ the potential temperature. If water vapor in all phases is admitted to the system, wet-bulb potential temperature or a similar conservative temperature may be used.