An instability of an unbounded parallel shear flow to the growth and nonlinear development of waves with phase speed in the along flow direction approximately equal to the speed of the inflexion point of the shear. This is also referred to as shear instability and is a specific example of barotropic instability. The instability, whether in homogeneous or stratified fluid, occurs due to a resonant coupling between wave-like disturbances on either flank of the shear flow where the gradient of the shear is non-zero. Increasing stratification acts to inhibit the growth of the instability.