alternate

[1]	rotate , exchange , change , interchange , take turns , go/take , etc in turn , (US) change off , interexchange
[2]	succeed , jump , be in succession/rotation
[3]	understudy
[4]	tack , switch , flip , flip-flop

[1]	surrogate , replacement
[2]	variant , alternative , (second) choice , (US/Canadian) substitute , deputy , stand-in , backup , understudy , pinch-hitter , Baseball designated hitter

adj.

[1]	in rotation , successive , every other , every second
[2]	alternative , second , other
[3]	substitute
[4]	alternating