(a.) Moving in the manner of undulations, or waves; resembling the motion of waves, which successively rise or swell rise or swell and fall; pertaining to a propagated alternating motion, similar to that of waves.

advocate agent alter ego alternating alternative amicus curiae analogy attorney back and fill back-and-forth backup backup man bandy battledore and shuttlecock be here again be quits with beating champion change changeling circle circling come again come and go come around come round come round again come up again commute comparison compensate complementary cooperate copy corresponding counterchange counterfeit cover cycle cyclic deputy dither do a hitch do a stint do a tour do time double dummy ebb and flow enlist epochal equal equivalent equivocate ersatz even every other exchange exchangeable executive officer exponent fake figurehead fill in for fill-in flounder fluctuate get back at get even with ghost ghostwriter give and take go through phases have a go have tenure hitch and hike hold office imitation in rotation interchange interchangeable intermit intermittent isochronal isochronous keep a watch lieutenant locum locum tenens logroll makeshift measured metaphor metonymy metronomic mock next best thing oscillate oscillatory other paranymph pass and repass pay back pendulate periodic periodical permute personnel phony pinch pinch hitter pleader procurator provisional proxy pulsate pulse pulsing re-up reappear reciprocal reciprocate reciprocative recur recurrent recurring reenlist relief relieve reoccur repeat replacement replacing representative requite reserve reserves respond retaliate return return the compliment revolve rhythmic ride and tie ring the changes ringer roll around rotary rotate rotating seasonal second second in command second string secondary seesaw serial serve time shift shilly-shally shuffle shuttle shuttlecock sign sign up sine wave spare spares spell spell off stagger stand-in steady stopgap sub substituent substitute substitution succedaneum succeed successive superseder supplanter surrogate swap sway swing switch symbol synecdoche take turns teeter teeter-totter temporary tentative tergiversate third string time off to-and-fro token totter trade transpose turn understudy undulant undulate undulatory up-and-down utility utility man utility player vacillate variant vary vicar vicar general vicarious vice vice-president vice-regent vicegerent wavelike waver wax and wane wheel wheel around wheeling wibble-wabble wigwag wobble zigzag