a student who attends a course but does not take it for credit
"The senior auditor attended every lecture on advanced calculus without receiving any academic credits."
a qualified accountant who inspects the accounting records and practices of a business or other organization
"The external auditor reviewed the company's financial statements to ensure they complied with all accounting standards."
One who audits bookkeeping accounts.
"The company hired an external auditor to review their financial records and ensure accuracy in the bookkeeping accounts."
In plain English: An auditor is someone who checks if money and records are correct.
"The company auditor reviewed our financial records to ensure everything was accurate before filing the report."
The word auditor comes from the Old French term auditour, which was borrowed into English from the Latin audītor. Originally meaning "one who hears," it described a person responsible for listening to accounts or reports.