What is the boundary between two different air masses called?

The boundary between two different air masses is called a weather front.

Air masses can differ in temperature, humidity, and density. When they meet, the denser air tends to remain near the surface while the warmer or moister air is lifted. Rising air cools, and that cooling can produce clouds, rain, snow, or thunderstorms.

A cold front forms when colder air advances and undercuts warmer air. A warm front forms when warmer air moves over retreating colder air. Stationary fronts have little movement, while occluded fronts develop when a cold front catches a warm front. Front symbols on weather maps help forecasters show these boundaries.