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.
      [1913 Webster]

            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.
         [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "acquaintance":
   account, acquaintedness, advocate, alter ego, amigo, announcement,
   appreciation, apprehension, associate, awareness, backer,
   best friend, blue book, bosom friend, briefing, brother, bulletin,
   casual acquaintance, close acquaintance, close friend, colleague,
   communication, communique, companion, comrade, confidant,
   confidante, consciousness, corpus, crony, data, datum, directory,
   dispatch, enlightenment, evidence, experience, expertise, facts,
   factual base, factual information, familiar, familiarity,
   familiarization, favorer, fellow, fellow creature, fellowman,
   friend, gen, general information, grasp, guidebook, handout,
   hard information, incidental information, info, information,
   inseparable friend, instruction, intelligence, intimacy, intimate,
   introduction, inwardness, ken, knockdown, know-how, knowing,
   knowledge, light, lover, mate, mention, message, neighbor, notice,
   notification, other self, partisan, pickup, practical knowledge,
   presentation, private knowledge, privity, promotional material,
   proof, publication, publicity, ratio cognoscendi, release, report,
   repository, self-knowledge, sidelight, statement, supporter,
   sympathizer, technic, technics, technique, the dope, the goods,
   the know, the scoop, transmission, understanding, well-wisher,
   white book, white paper, word



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.