Definitions for "Potential temperature"
A physical oceanographic term for the temperature that a water sample gathered at depth would have if brought adiabatically (i.e. without thermal contact with the surrounding water) to the surface, i.e. the effective temperature of a water parcel after removing the heat of the parcel associated solely with compression. A sample brought from depth to the surface will, due to the slight compressibility of sea water, expand and therefore tend to cool, and as such potential temperatures at great depths are always less than measured temperatures.
The potential temperature is the temperature a parcel of water would have if it was at the surface. The potential temperature takes into account the temperature change that occurs whenever the pressure of the water changes. The potential temperature is always less than the in situ temperature.
The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.