From AMS Glossary
(Also called autoconvective instability.) The supposed spontaneous initiation of convection in an atmospheric layer in which density increases with height and in which the lapse rate is greater than the autoconvective lapse rate.
The term is based on a false analogy with convection in an incompressible fluid. Convection in a gas will initiate spontaneously, through buoyant instability, if the lapse rate is greater than the adiabatic lapse rate.