From AMS Glossary
A semipermanent feature of the atmospheric circulation (usually a high-pressure ridge) on opposite sides of which the prevailing wind directions differ greatly.
A good example is the ridge that extends in winter from the Siberian high westward across central Europe and France, and in summer less regularly from the Azores high across Spain and France to central Europe. North of this the prevailing winds are from west-southwest, while south of it they are on the whole northeasterly. Köppen speaks of the polar wind divide, a very diffuse boundary of low pressure between the midlatitude westerlies and the polar easterlies.