"following each other by turns, reciprocal," 1510s, from Latin alternatus "one after the other," past participle of alternare "to do first one thing then the other; exchange parts," from alternus "one after the other, alternate, in turns, reciprocal," from alter "the other" (see alter).
Alternate means "by turns;" alternative means "offering a choice." Both imply two kinds or things. Alternation is the process of two things following one another regularly by turns (as night and day); an alternative is a choice of two things, the acceptance of one implying the rejection of the other. Related: Alternacy.