Nouns

(n)surrogate, alternate, replacementsomeone who takes the place of another person

Verbs

(v)alternate, jumpgo back and forth; swing back and forth between two states or conditions
(v)alternateexchange people temporarily to fulfill certain jobs and functions
(v)understudy, alternatebe an understudy or alternate for a role
(v)interchange, tack, switch, alternate, flip, flip-flopreverse (a direction, attitude, or course of action)
(v)alternate, take turnsdo something in turns“We take turns on the night shift”

Adjectives

(s)alternateevery second one of a series“the cleaning lady comes on alternate Wednesdays”
(s)alternate, alternative, substituteserving or used in place of another“an alternative plan”
(s)alternate, alternatingoccurring by turns; first one and then the other“alternating feelings of love and hate”
(a)alternateof leaves and branches etc.; first on one side and then on the other in two ranks along an axis; not paired“stems with alternate leaves”