either

from The Century Dictionary.

• In one case; according to one choice or supposition (in a series of two or more): a disjunctive conjunction, preceding one of a series of two or more alternative clauses, and correlative with or before the following clause or clauses. Sometimes, as in poetry, or is used before the first clause also.
• In any case; at all: used adverbially, for emphasis, after a sentence expressing a negation of one or two alternatives, or of all alternatives: corresponding to too similarly used after affirmative sentences: as, he tried it, and didn't succeed; then I tried it, but I didn't succeed, either. That's mine; no, it isn't, either.
• Being one or the other of two, taken indifferently or as the case requires: referring to two units or particulars of a class: as, it can be done in either way; take either apple; the boat will land on either side.
• Being one and the other of two; being both of two, or each of two taken together but viewed separately: as, they took seats on either side.
• [In this use, each or both, according to construction, is nearly if not quite always to be preferred. Properly, either refers indefinitely to one or the other of two (and often in actual use, though less accurately, to some one of any number); each, definitely to every one of two or any larger number considered individually: a distinctness of signification which ought to be maintained, since interchange of the words (less practised by careful writers now than formerly) offers no advantage, but may create ambiguity. Both, two together, one and the other taken jointly, should be preferred when this is the specific sense; but both and each may often be interchanged. Thus, the camp may be pitched on either side of the stream (on one or the other side indifferently); there were two camps, one on each side; the camp was pitched on both sides (one camp, divided); there are fine buildings on both sides of the street, or on each side, but not on either side.]
• One or the other; one of two, taken indifferently.
• Each of two; the one and the other.

from the GNU version of the Collaborative International Dictionary of English.

• adjective One of two; the one or the other; -- properly used of two things, but sometimes of a larger number, for any one.
• adjective Each of two; the one and the other; both; -- formerly, also, each of any number.
• conjunction Either precedes two, or more, coördinate words or phrases, and is introductory to an alternative. It is correlative to or.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• pronoun obsolete Both, each of two (people or things).
• pronoun One or other of two people or things.
• adverb conjunctive, after a negative as well
• conjunction Introduces the first of two options, the second of which is introduced by "or".

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adverb after a negative statement used as an intensive meaning something like `likewise' or `also'