An audit trail is a record of transactions or communications all related to a single person, account or other entity.

Webopedia calls it:

"A record showing who has accessed a computer system and what operations he or she has performed during a given period of time." [1] (http://www.webopedia.com/TERM/A/audit_trail.html)

In telecommunication, the term audit trail has the following meanings:

1. A record of both completed and attempted accesses and service.

2. Data in the form of a logical path linking a sequence of events, used to trace the transactions that have affected the contents of a record.

3. In INFOSEC, a chronological record of system activities to enable the reconstruction and examination of the sequence of events and/or changes in an event.

Note: Audit trail may apply to information in an AIS, or to the transfer of COMSEC material.

Source: from Federal Standard 1037C and from the National Information Systems Security Glossary

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In accounting, documentation of detailed transactions supporting summary ledger entries. The documentation may be on paper or electronic records.