Foundations
Log File
Also: log, access log
A text record a program or operating system appends to, usually one line per event with a timestamp.
In more detail
A useful log line answers when, who or what, did what, and with what result. Logs are the main evidence after an incident and the input to most detection. They can hold secrets and personal data, can be edited by an intruder, and show only what reached them, so keep keys out of them and copy them somewhere the machine cannot alter.