au·di·tor 
(ôdĭ-tər)
(ôdĭ-tər)n.
1. One that audits accounts.
2. One who audits a course.
3. One who hears; a listener.
[Middle English, from Anglo-Norman auditour, from Latin audītor, listener, from audīre, to hear; see au- in the Appendix of Indo-European roots.]