An audit trail is the chronological record of actions in a system: who signed in, who changed a setting, who exported data, and when. Good audit trails are complete, tamper-evident and retained long enough to support investigations and compliance reviews.
In workplace systems the audit trail answers the awkward questions: who let this visitor in, who changed the booking rules, who downloaded the attendance report. Frameworks from ISO 27001 to SOC 2 expect them, and enterprise buyers increasingly ask to see what the trail captures before they sign.