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.

1590s, from Latin alternatus, past participle of alternare (see alternate, adj.). Replaced Middle English alternen "to vary, alternate" (early 15c.). Related: Alternated; alternating.

1718, "that which alternates (with anything else)," from alternate, adj.. Meaning "a substitute" is first attested 1848.