What is the term for a boundary separating two airmasses of different densities?

The term used to describe a boundary that separates two air masses of different densities is "Front." In meteorology, a front is typically associated with changes in temperature, humidity, and wind direction. When two air masses converge, such as cold air meeting warm air, they do not mix immediately due to their differing densities. This results in a front, which can give rise to various weather phenomena, including precipitation, storms, and changes in weather patterns.

The concept of fronts is crucial for understanding weather forecasting and the dynamics of the atmosphere. A front can be cold, warm, stationary, or occluded, each representing different interactions between air masses. Through this understanding, one can predict changes in weather conditions.

The other terms do not accurately describe this specific meteorological phenomenon. For instance, a "Zone" is a more general term that doesn’t specifically relate to the concept of air mass boundaries. "Stall" refers to the loss of lift in an aircraft rather than air mass interactions. The "Boundary layer" is a term used to describe a layer of the atmosphere directly influenced by the Earth's surface, and while it plays an important role in atmospheric dynamics, it is not the correct term for the boundary between air masses.