From AMS Glossary
(Also called convective instability, thermal instability.) The state of an unsaturated layer or column of air in the atmosphere with a wet-bulb potential temperature (or equivalent potential temperature) that decreases with elevation.
If such a column is lifted bodily until completely saturated, it will become unstable (i.e., its temperature lapse rate will exceed the saturation-adiabatic lapse rate) regardless of its initial stratification.
Saucier, W. J. 1955. Principles of Meteorological Analysis. 76–78.