( ( front ( ( The boundary ) ( between ( ( two ( air masses ) ) ( of ( different ( ( temperature and ) density ) ) ) ) ) ) ) . )
(ROOT (NP (NP (NN front)) (NP (NP (DT The) (NN boundary)) (PP (IN between) (NP (NP (CD two) (NN air) (NNS masses)) (PP (IN of) (NP (JJ different) (NN temperature) (CC and) (NN density)))))) (. .)))
front The boundary between two air masses of different temperature and density.
(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.