( Fronts ( ( mark ( ( the boundary ) ( between ( two ( air masses ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Fronts)) (VP (VBP mark) (NP (NP (DT the) (NN boundary)) (PP (IN between) (NP (CD two) (NN air) (NNS masses))))) (. .)))
Fronts mark the 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.