The Collaborative International Dictionary of English v.0.48:

Customer \Cus"tom*er\ (k[u^]s"t[u^]m*[~e]r), n. [A doublet of
   customary, a.: cf. LL. custumarius toll gatherer. See
   Custom.]
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   1. One who collect customs; a toll gatherer. [Obs.]
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            The customers of the small or petty custom and of
            the subsidy do demand of them custom for kersey
            cloths.                               --Hakluyt.
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   2. One who regularly or repeatedly makes purchases of a
      trader; a purchaser; a buyer.
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            He has got at last the character of a good customer;
            by this means he gets credit for something
            considerable, and then never pays for it.
                                                  --Goldsmith.
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   3. A person with whom a business house has dealings; as, the
      customers of a bank. --J. A. H. Murray.
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   4. A peculiar person; -- in an indefinite sense; as, a queer
      customer; an ugly customer. [Colloq.] --Dickens.
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   5. A lewd woman. [Obs.] --Shak.
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WordNet (r) 3.0 (2006):

customer
    n 1: someone who pays for goods or services [syn: customer,
         client]