either

adjective

1. one or the other (of two) use either hand
2. each (of two); the one and the other he had a tool in either hand

Etymology: ME < OE æghwæther < a (æ), always (see ay) + gehwæther, each of two (see whether): akin to, and of same formation as, OHG eogihwedar

one or the other (of two)

the first element of the pair of disjunctive correlatives either … or, implying a choice of alternatives [either go or stay]: either … or is sometimes used to refer to more than two, although this use is objected to by some bring either cookies, doughnuts, or some other kind of pastry

adverb

1. any more than the other; also: used after negative expressions if you don't go, I won't either
2. Informal certainly; indeed: used as an intensifier in a negative statement “It's mine.”“It isn't either!”