Quiz: Remember the definition of mal de mer?
1550s, present participle adjective from alternate (v.). Alternating current is recorded from 1839.
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.
1718, "that which alternates (with anything else)," from alternate (adj.). Meaning "a substitute" is first attested 1848.