From AMS Glossary
A theory originated by the Scandinavian school of meteorologists whereby a polar front, separating air masses of polar and tropical origin, gives rise to cyclonic disturbances that intensify and travel along the front, passing through various phases of a characteristic life history.
This theory ushered in a new era of atmospheric analysis and remains an important basis of practical synoptic meteorology and weather forecasting even today.
Bjerknes, J., and Solberg, H. 1922. Life cycles of cyclones and polar front theory of atmospheric circulation. Geofys. Publ.. 3. 3–18.