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]

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

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