auditor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that audits accounts.
• noun One who audits a course.
• noun One who hears; a listener.

from The Century Dictionary.

• noun A hearer; one who listens to what is said; a member of an auditory.
• noun Same as audient, n.
• noun A person appointed and authorized to examine an account or accounts, compare the charges with the vouchers, examine parties and witnesses, allow or reject charges, and state the result.
• noun One of certain officers of high rank at the papal court: so called from their connection with business treated of in audiences with the pope: as, auditor of the apostolic chamber; auditor of the pope; auditors of the Roman rota (which see).

from the GNU version of the Collaborative International Dictionary of English.

• noun A hearer or listener.
• noun 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.
• noun One who hears judicially, as in an audience court.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who audits bookkeeping accounts.
• noun In many jurisdictions, an elected or appointed public official in charge of the public accounts; a comptroller.
• noun One who audits an academic course; who attends the lectures but does not earn academic credit.
• noun rare One who listens as a member of an audience
• noun Scientology One trained to perform spiritual guidance procedures.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a student who attends a course but does not take it for credit
• noun a qualified accountant who inspects the accounting records and practices of a business or other organization
• noun someone who listens attentively