The Collaborative International Dictionary of English v.0.48:

Acquaintance \Ac*quaint"ance\, n. [OE. aqueintance, OF.
   acointance, fr. acointier. See Acquaint.]
   1. A state of being acquainted, or of having intimate, or
      more than slight or superficial, knowledge; personal
      knowledge gained by intercourse short of that of
      friendship or intimacy; as, I know the man; but have no
      acquaintance with him.
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            Contract no friendship, or even acquaintance, with a
            guileful man.                         --Sir W.
                                                  Jones.
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   2. A person or persons with whom one is acquainted.
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            Montgomery was an old acquaintance of Ferguson.
                                                  --Macaulay.
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   Note: In this sense the collective term acquaintance was
         formerly both singular and plural, but it is now
         commonly singular, and has the regular plural
         acquaintances.
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   To be of acquaintance, to be intimate.

   To take acquaintance of or with, to make the acquaintance
      of. [Obs.]
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   Syn: Familiarity; intimacy; fellowship; knowledge.

   Usage: Acquaintance, Familiarity, Intimacy. These words
          mark different degrees of closeness in social
          intercourse. Acquaintance arises from occasional
          intercourse; as, our acquaintance has been a brief
          one. We can speak of a slight or an intimate
          acquaintance. Familiarity is the result of continued
          acquaintance. It springs from persons being frequently
          together, so as to wear off all restraint and reserve;
          as, the familiarity of old companions. Intimacy is the
          result of close connection, and the freest interchange
          of thought; as, the intimacy of established
          friendship.
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                Our admiration of a famous man lessens upon our
                nearer acquaintance with him.     --Addison.
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                We contract at last such a familiarity with them
                as makes it difficult and irksome for us to call
                off our minds.                    --Atterbury.
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                It is in our power to confine our friendships
                and intimacies to men of virtue.  --Rogers.
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WordNet (r) 3.0 (2006):

acquaintance
    n 1: personal knowledge or information about someone or
         something [syn: acquaintance, familiarity,
         conversance, conversancy]
    2: a relationship less intimate than friendship [syn:
       acquaintance, acquaintanceship]
    3: a person with whom you are acquainted; "I have trouble
       remembering the names of all my acquaintances"; "we are
       friends of the family" [syn: acquaintance, friend] [ant:
       alien, stranger, unknown]

The Devil's Dictionary (1881-1906):

ACQUAINTANCE, n.  A person whom we know well enough to borrow from,
but not well enough to lend to.  A degree of friendship called slight
when its object is poor or obscure, and intimate when he is rich or
famous.