WordReference Random House Learner's Dictionary of American English © 2015
ei•ther /ˈiðɚ, ˈaɪðɚ/USA pronunciation adj. [+ singular countable noun]
  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.

  • one or the other: Either will do.

  • conj. 
  • (used with or to indicate a series of choices): Either call or write.

  • adv. 
  • (used with a negative word, phrase, or clause) as well;
    likewise: If you don't go, I won't either.
  • When used as the subject, the pronoun either takes a singular verb even when followed by a prepositional phrase with a plural object: Either of the shrubs grows well in this soil. As an adjective either refers only to two of anything. As a conjunction, either often introduces a series of more than two: pizza topped with either onions, peppers, or mushrooms. Usage guides say that when subjects are joined by eitheror (or neithernor), the verb is singular or plural depending on the noun or pronoun nearer the verb: Either the parents or the school determines the program. Either the school or the parents determine the program. See also neither.

    Collins Concise English Dictionary © HarperCollins Publishers::

    either /ˈaɪðə; ˈiːðə/ determiner
    1. one or the other (of two): either coat will do
    2. (as pronoun): either is acceptable
    3. both one and the other: there were ladies at either end of the table
    4. (coordinating) used preceding two or more possibilities joined by "or": you may have either cheese or a sweet
    adv (sentence modifier)
    1. (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
    Etymology: Old English ǣgther, short for ǣghwæther each of two; related to Old Frisian ēider, Old High German ēogihweder; see each, whether
    Either is followed by a singular verb in good usage: either is good; either of these books is useful. Care should be taken to avoid ambiguity when using either to mean both or each, as in the following sentence: a ship could be moored on either side of the channel. Agreement between the verb and its subject in either…or… constructions follows the pattern given for neither…nor…

    'either' also found in these entries:

    Forum discussions with the word(s) "either" in the title:

    Look up "either" at Merriam-Webster
    Look up "either" at dictionary.com

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