From AMS Glossary
A hydrodynamic instability arising from the existence of a meridional temperature gradient (and hence vertical shear of the mean flow and a thermal wind) in an atmosphere in quasigeostrophic equilibrium and possessing static stability.
For reasonable values of the atmospheric parameters, the wavelength of maximum instability corresponds to that of a synoptic-scale disturbance. Such a system may be interpreted as converting potential energy of the basic flow into kinetic energy of the unstable perturbation.
Holton, J. R. 1992. An Introduction to Dynamic Meteorology. 3d edition, Academic Press, . 228–264.