The Collaborative International Dictionary of English v.0.48:

Auditor \Au"di*tor\, n. [L. auditor, fr. audire. See Audible,
   a.]
   1. A hearer or listener. --Macaulay.
      [1913 Webster]

   2. A person appointed and authorized to audit or examine an
      account or accounts, compare the charges with the
      vouchers, examine the parties and witnesses, allow or
      reject charges, and state the balance.
      [1913 Webster]

   3. One who hears judicially, as in an audience court.
      [1913 Webster]

   Note: In the United States government, and in the State
         governments, there are auditors of the treasury and of
         the public accounts. The name is also applied to
         persons employed to check the accounts of courts,
         corporations, companies, societies, and partnerships.
         [1913 Webster]

WordNet (r) 3.0 (2006):

auditor
    n 1: someone who listens attentively [syn: hearer, listener,
         auditor, attender]
    2: a student who attends a course but does not take it for
       credit
    3: a qualified accountant who inspects the accounting records
       and practices of a business or other organization

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "auditor":
   Big Brother, CA, CPA, accepter, accountant, accountant general,
   acquirer, actuary, addressee, audience, audient, autodidact,
   bank accountant, bank examiner, beholder, boatswain, bookkeeper,
   boss, bursar, calculator, cashier, cashkeeper,
   certified public accountant, chamberlain, chartered accountant,
   chief, clerk, comptroller, consignee, controller, cost accountant,
   cost keeper, curator, depositary, depository, eavesdropper,
   educatee, financial officer, floor manager, floorman, floorwalker,
   foreman, gaffer, ganger, getter, head, headman, hearer, hearkener,
   holder, inquirer, inspector, journalizer, learner, liquidator,
   listener, looker, monitor, noncommissioned officer, obtainer,
   overhearer, overman, overseer, payee, paymaster, praepostor,
   prefect, proctor, procurer, pupil, purse bearer, purser, receiver,
   recipient, reckoner, recorder, registrar, scholar,
   self-taught person, sirdar, slave driver, snoop, spectator,
   steward, straw boss, student, studier, subforeman, super,
   superintendent, supervisor, surveyor, taker, taskmaster, trainee,
   treasurer, trustee, viewer, visitor