From AMS Glossary
An extratropical surface cyclone with a central pressure that falls on the average at least 1 mb h-1 for 24 hours.
This predominantly maritime, cold season event is usually found approximately 750 km downstream from a mobile 500-mb trough, within or poleward of the maximum westerlies, and within or ahead of the planetary-scale troughs.
Sanders, F., and J. R. Gyakum 1980. Synoptic–dynamic climatology of the "bomb". Mon. Wea. Rev.. 108. 1589–1606.