When two air masses meet, the air within them does not easily mix. That is to say that the air in one air mass will not easily mix with the air from another air mass. Instead, the air stays within its own air mass. Because of this phenomena, a border forms between two clashing air masses as they rub together. This border is called a front.
The term ‘front’ was first used by Norwegian scientists who compared two air masses colliding to two armies as they clash on the battle front. Just like one army taking over another in a battle, one air mass ultimately takes over, pushing the other one away.
Fronts should not be thought of as a vertical wall. This is because they are actually sloped, like a hill. Often the slope of a front can be extremely gradual, taking hundreds of miles across the surface of the Earth to reach an altitude of just one mile.
While the air in one air mass will not mix easily with the air from another air mass, along a front there is some mixing. The front where air mixes is usually just a few miles or tens of miles across.