alternate (adj.) Look up alternate at Dictionary.com
"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. Related: Alternacy.
alternate (v.) Look up alternate at Dictionary.com
1590s, "do by turns" (transitive), from Latin alternatus, past participle of alternare "do one thing and then another, do by turns," from alternus "one after the other, alternate, in turns, reciprocal," from alter "the other" (see alter). Replaced Middle English alternen "to vary, alternate" (early 15c.). Transitive meaning "interchange reciprocally" is from 1850; intransitive sense "follow one another in time or place" is from c. 1700; that of "pass back and forth between actions, conditions, etc." is by 1823. Related: Alternated; alternating.
alternate (n.) Look up alternate at Dictionary.com
1718, "that which alternates (with anything else)," from alternate (adj.). Meaning "a substitute, one authorized to take the place of another," especially in political bodies, is first attested 1848.