( ( ( Front - ) ( ( The ( boundary ( or ( transition zone ) ) ) ) ( between ( two ( different ( air masses ) ) ) ) ) ) . )
(ROOT (NP (NP (NNP Front)) (: -) (NP (NP (DT The) (NN boundary) (CC or) (NN transition) (NN zone)) (PP (IN between) (NP (CD two) (JJ different) (NN air) (NNS masses)))) (. .)))
Front- The boundary or transition zone between two 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.