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.]
   [1913 Webster]
   1. One who collect customs; a toll gatherer. [Obs.]
      [1913 Webster]

            The customers of the small or petty custom and of
            the subsidy do demand of them custom for kersey
            cloths.                               --Hakluyt.
      [1913 Webster]

   2. One who regularly or repeatedly makes purchases of a
      trader; a purchaser; a buyer.
      [1913 Webster]

            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.
      [1913 Webster]

   3. A person with whom a business house has dealings; as, the
      customers of a bank. --J. A. H. Murray.
      [1913 Webster]

   4. A peculiar person; -- in an indefinite sense; as, a queer
      customer; an ugly customer. [Colloq.] --Dickens.
      [1913 Webster]

   5. A lewd woman. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "customer":
   Adamite, being, bloke, body, buyer, cat, chap, character, client,
   consumer, creature, duck, earthling, fellow, groundling, guy, hand,
   head, homo, human, human being, individual, joker, life,
   living soul, man, mark, mortal, nose, one, party, patron, person,
   personage, personality, prospect, purchaser, regular,
   regular customer, shopper, single, somebody, someone, soul, sucker,
   tellurian, terran, worldling