adjective
one or the other of two:
You may sit at either end of the table.
each of two; the one and the other:
There are trees on either side of the river.
pronoun
one or the other:
There are two roads into the town, and you can take either. Either will do.
conjunction
(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
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.