Definition of: either

(ē′thər, ī′-) adjective
1. One or the other of two: Use either foot on the pedal.
2. Each of two; one and the other: on either side. See synonyms under EVERY.
—pron. One of two; one or the other.
—conj. In one of two or more cases, indeterminately or indifferently: used as a disjunctive correlative introducing a first alternative, the second and any other being preceded by or: Either I shall go or he will come.
—adverb At all; any more so: used after the denial of an alternative: He could not speak, and I could not either. [OE œgther]