(meteorology) the atmospheric phenomenon created at the boundary between two different air masses WordNet 3.0 & Open English WordNet

A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Wikipedia

A boundary separating air masses, and the cause of most weather phenomena Wikipedia Disambiguation

Boundary separating two masses of air of different densities Wikidata

The line of contact of air masses of different temperatures due varying density. OmegaWiki

The interface or transition zone between two airmasses of different density, often resulting in precipitation. Since the temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates airmasses of different temperature. Wiktionary

Meteorology: interface between airmasses. Wiktionary (translation)

The boundary between two masses of air of different density and/or temperature. Wiktionary

Boundary between two masses of air. Wiktionary (translation)