Either

[ee-th er, ahy-th er] /ˈi ðər, ˈaɪ ðər/

adjective
1.
one or the other of two:
You may sit at either end of the table.
2.
each of two; the one and the other:
There are trees on either side of the river.
pronoun
3.
one or the other:
There are two roads into the town, and you can take either. Either will do.
conjunction
4.
(a coordinating conjunction that, when preceding a word or statement followed by the disjunctive or, serves to emphasize the possibility of choice):
Either come or write.
adverb
5.
also; too; as well; to the same degree (used after negative clauses coordinated by and, or, or nor, or after negative subordinate clauses):
He’s not fond of parties, and I’m not either. If you don’t come, she won’t come either.
/ˈaɪðə; ˈiːðə/
determiner
1.

2.
both one and the other: there were ladies at either end of the table
3.
(coordinating) used preceding two or more possibilities joined by “or”: you may have either cheese or a sweet
adverb (sentence modifier)
4.
(used with a negative) used to indicate that the clause immediately preceding is a partial reiteration of a previous clause: John isn’t a liar, but he isn’t exactly honest either
adj.

Old English ægðer, contraction of æghwæðer “each of two, both,” from a “always” (see aye (adv.)) + ge- collective prefix + hwæðer “which of two, whether” (see whether).

Cognate with Dutch ieder, Old High German eogiwedar, German jeder “either, each, every”). Modern sense of “one or the other of two” is late 13c. Use of either-or to suggest an unavoidable choice between alternatives (1931) in some cases reflects Danish enten-eller, title of an 1843 book by Kierkegaard.