substitute

[1]	substitute for , replace , exchange , displace , relieve , supplant , switch , take the place of , stand in for , double for , (Colloq) sub for , cover for , swap/swop , (US/Canadian) pinch-hit for
[2]	replace , interchange , exchange
[3]	sub , stand in , fill in
[4]	deputize , deputise , step in

[1]	replacement
[2]	reserve , substitution , replacement , second-stringer , alternative , relief , representative , deputy , delegate , stand-in , stand-by , understudy , surrogate , succedaneum , (Brit) locum (tenens) , (US/Canadian) alternate
[3]	stand-in , relief , reliever , backup , backup man , fill-in

adj.