What is 'bomb cyclone' that's causing temperatures as low as -48°C in the US?

A winter storm, that brought dangerously low temperatures of -48°C and snow as deep as 43 inches in the US causing over 30 deaths, is called a "bomb cyclone". It occurs when a mass of low-pressure air meets a high-pressure mass. In bomb cyclone or bombogenesis, pressure drops in the low-pressure mass by at least 24 millibars in 24 hours.