What is the definition of a cold front?

Dear Tom,
What is the definition of a cold front?
— David Greenberg,
Highland Park
Dear David,
In meteorology, a “front” is the transition zone between two air masses of different temperature and density. A cold front (at the ground) marks the leading edge of advancing colder air (replacing warmer air) and a warm front (at the ground) marks the leading edge of advancing warmer air (replacing colder air).
The transition from warmer to cooler air can be abrupt, a matter of a few miles, or it can be gradual, over perhaps a hundred miles or more. If the frontal boundary is moving very little or not at all, it is called a “stationary front.” In the Midwest, cold fronts usually produce precipitation along or ahead of them: showers and thunderstorms in the warm season and rain or snow in the cold season.
A cold front can vary in size and can help predict storms and snow in the Midwest. But by meteorological definition, it’s the transition zone between air masses.