( ( ( The ( half life ) ) ( being ( the time ) ) ) ( it ( ( ( takes ( for ( ( one half ) ( of ( ( the material ) ( originally present ) ) ) ) ) ) ( to decay ) ) . ) ) )
(ROOT (S (S (NP (DT The) (NN half) (NN life)) (VP (VBG being) (NP (DT the) (NN time)))) (NP (PRP it)) (VP (VBZ takes) (PP (IN for) (NP (NP (CD one) (NN half)) (PP (IN of) (NP (NP (DT the) (NN material)) (ADJP (RB originally) (JJ present)))))) (PP (TO to) (NP (NN decay)))) (. .)))
The half life being the time it takes for one half of the material originally present to decay.
(ROOT (S (NP (DT The) (NN term) (NN half-life)) (VP (VBZ decribes) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NP (NN time)) (VP (VBN required) (PP (IN for) (NP (NP (NN half)) (PP (IN of) (NP (DT the) (JJ original) (NN material))))) (PP (TO to) (NP (NP (NN decay)) (PP (IN in) (NP (DT an) (NN isotope)))))))))) (. .)))