( ( A front ) ( ( is ( ( the boundary ) ( between ( ( two ( different masses ) ) ( of air ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN front)) (VP (VBZ is) (NP (NP (DT the) (NN boundary)) (PP (IN between) (NP (NP (CD two) (JJ different) (NNS masses)) (PP (IN of) (NP (NN air))))))) (. .)))
A front is the boundary between two different masses of air.
(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.