Audit log
A record of who did what, when, in a system, retained so it can be reviewed later.
Audit logs answer the questions that matter during an investigation: who accessed the record, who changed the permission, who signed in from where. Without them, an incident response is largely guesswork.
Retention is the decision that determines their usefulness. Default retention in many cloud services is shorter than the typical time to discover a breach, which means the evidence is gone before anyone looks for it.
Where this comes up
Audit log sits inside our compliance and frameworks work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.