(1):
(a. & pron.) Each of two; the one and the other; both; - formerly, also, each of any number.
(2):
(a. & pron.) One of two; the one or the other; - properly used of two things, but sometimes of a larger number, for any one.
(3):
(conj. Either) precedes two, or more, coordinate words or phrases, and is introductory to an alternative. It is correlative to or.