What is Audit Trail?

Legal Definition
An audit trail (also called audit log) is a security-relevant chronological record, set of records, and/or destination and source of records that provide documentary evidence of the sequence of activities that have affected at any time a specific operation, procedure, or event. Audit records typically result from activities such as financial transactions, scientific research and health care data transactions, or communications by individual people, systems, accounts, or other entities.

The process that creates an audit trail is typically required to always run in a privileged mode, so it can access and supervise all actions from all users; a normal user should not be allowed to stop/change it. Furthermore, for the same reason, trail file or database table with a trail should not be accessible to normal users. Another way of handling this issue is through the use of a role-based security model in the software. The software can operate with the closed-looped controls, or as a 'closed system', as required by many companies when using audit trail functionality.
-- Wikipedia
Legal Definition
A trail that shows a transactions history. It leads back to the source document. A clean trail is the sign of a well organized company.