( ( A front ) ( ( is ( ( a boundary ) ( between ( different ( air masses ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN front)) (VP (VBZ is) (NP (NP (DT a) (NN boundary)) (PP (IN between) (NP (JJ different) (NN air) (NNS masses))))) (. .)))
A front is a boundary between different air masses.
(ROOT (S (PP (IN In) (NP (NN weather) (NNS terms))) (, ,) (NP (NP (DT the) (NN boundary)) (PP (IN between) (NP (CD two) (NN air) (NNS masses)))) (VP (VBZ is) (VP (VBN called) (NP (NN front)))) (. .)))
In weather terms, the boundary between two air masses is called front.