What is "equivalent potential temperature"

n. The temperature a parcel of air at a specific temperature would have if raised from a specific pressure level to zero millibars, had all moisture condensed from it, and then lowered to 1000 millibars.

Equivalent potential temperature, commonly referred to as theta-e (θ), is a quantity related to the stability of a column of air in the atmosphere.

θ is the temperature a parcel of air would reach if all the water vapor in the parcel were to condense, releasing its latent heat, and the parcel was brought adiabatically to a standard reference pressure, usually 1000 hPa (1000 mbar) which is roughly equal to atmospheric pressure at sea level. In stable conditions, θ increases with altitude. If θ decreases with height, convection can occur. The comparison of the equivalent potential temperature of parcels of air at different pressures thus provides a measure of the instability of the column of air.