( ( ( ( A front ) ( -LRB- ( ( boundary ( between ( two ( air masses ) ) ) ) -RRB- ) ) ) ( that ( ( is not ) moving ) ) ) . )
(ROOT (NP (NP (DT A) (NN front)) (PRN (-LRB- -LRB-) (VP (VBN boundary) (PP (IN between) (NP (CD two) (NN air) (NNS masses)))) (-RRB- -RRB-)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (RB not) (VP (VBG moving))))) (. .)))
A front (boundary between two air masses) that is not moving.
(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.