( ( A front ) ( ( is ( defined ( as ( ( a ( narrow ( transition ( ( zone or ) boundary ) ) ) ) ( between ( two ( air masses ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN front)) (VP (VBZ is) (VP (VBN defined) (PP (IN as) (NP (NP (DT a) (JJ narrow) (NN transition) (NN zone) (CC or) (NN boundary)) (PP (IN between) (NP (CD two) (NN air) (NNS masses))))))) (. .)))
A front is defined as a narrow transition zone or boundary between two 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.